feat: refactor settings UI with category navigation and add Delimiters page

Closes #640

## Changes

### UI Refactoring
- Replace monolithic settings dialog with category-based navigation
- Add QListWidget (lw_categories) for category selection on the left
- Add QStackedWidget (sw_categories) for content pages on the right
- Organize settings into 5 distinct pages:
  - Flags (14 checkboxes)
  - Quality (3 combo boxes)
  - Numbers (2 spin boxes)
  - Paths & Formats (7 line edits + 2 browse buttons)
  - Delimiters (4 line edits) — NEW

### New Feature: Delimiters Category
- Add customizable separators for artists in metadata and filenames:
  - metadata_delimiter_artist
  - metadata_delimiter_album_artist
  - filename_delimiter_artist
  - filename_delimiter_album_artist
- Auto-integration with DialogPreferences via parameters_line_edit
- Line edits with max width 100px for compact display

### Styling
- Apply dark theme to settings dialog:
  - Category list: #2b2b2b background, #3d5a80 selection, #3a3a3a hover
  - Settings area: #333333 background, styled GroupBoxes (#3a3a3a)
  - Light text (#e0e0e0) for better contrast
- Add spacing property (6px) to category list for better readability
- Style QListWidget items with padding (4px 3px) and margin (2px 0px)

### Testing
- Add comprehensive test suite (61 tests total):
  - test_settings_ui.py: UI and basic integration (14 tests)
  - test_settings_dialog_structure.py: detailed structure validation (20 tests)
  - test_dialog_preferences_integration.py: DialogPreferences integration (14 tests)
  - test_delimiters_category.py: new Delimiters page validation (13 tests)
- Fix ruff issues (F841, S108) in test files
- All tests passing with 100% success rate

### Documentation
- Add docs/refactor_settings_ui.md: comprehensive refactor overview
- Add docs/delimiters_category.md: Delimiters feature documentation
- Add docs/ui_styles.md: dark theme style guide
- Add docs/testing_summary.md: testing overview and commands

### Code Quality
- Apply ruff/black/pyupgrade via pre-commit hooks
- Fix F841 (unused variable) in test_dialog_preferences_integration.py
- Fix S108 (insecure temp path) using Path.home() in test_settings_ui.py
- End-of-file fixes applied automatically

## Files Modified
- tidal_dl_ng/ui/dialog_settings.ui (complete restructure)
- tidal_dl_ng/ui/dialog_settings.py (auto-generated from .ui)
- tidal_dl_ng/dialog.py (add 4 delimiter params + Delimiters category)
- tests/* (4 new test files with 61 tests)
- docs/* (4 new documentation files)

## Impact
- Improved UX: intuitive category navigation, better visual organization
- Extensibility: easy to add new settings pages/categories
- Quality: comprehensive test coverage ensures robustness
- User flexibility: customizable separators for metadata and filenames

## Technical Notes
- QStackedWidget pages indexed 0-4 matching category list order
- Navigation via QListWidget::currentRowChanged signal
- Automatic save/load through existing DialogPreferences methods
- No breaking changes to existing settings or configuration

## Testing Commands
```powershell
poetry run pre-commit run -a
poetry run pytest -q

1 files changed, 257 insertions, 0 deletions

diff --git a/tests/test_delimiters_category.py b/tests/test_delimiters_category.py
new file mode 100644
index 0000000..df5e950
--- /dev/null
+++ b/tests/test_delimiters_category.py
@@ -0,0 +1,257 @@
+"""Tests for the Delimiters category in settings dialog."""
+
+from unittest.mock import Mock, patch
+
+import pytest
+from PySide6 import QtWidgets
+
+from tidal_dl_ng.dialog import DialogPreferences
+from tidal_dl_ng.ui.dialog_settings import Ui_DialogSettings
+
+
+@pytest.fixture
+def qapp():
+    """Create QApplication instance for tests."""
+    app = QtWidgets.QApplication.instance()
+    if app is None:
+        app = QtWidgets.QApplication([])
+    yield app
+
+
+class TestDelimitersPage:
+    """Test the Delimiters page specifically."""
+
+    def test_delimiters_page_exists(self, qapp):
+        """Test that the Delimiters page exists."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        assert hasattr(ui, "page_delimiters")
+        assert ui.page_delimiters is not None
+
+    def test_delimiters_page_is_fifth(self, qapp):
+        """Test that the Delimiters page is at index 4."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        assert ui.sw_categories.widget(4) == ui.page_delimiters
+
+    def test_delimiters_groupbox_exists(self, qapp):
+        """Test that the Delimiters GroupBox exists."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        assert hasattr(ui, "gb_delimiters")
+        assert ui.gb_delimiters is not None
+        assert isinstance(ui.gb_delimiters, QtWidgets.QGroupBox)
+
+    def test_delimiters_page_has_four_line_edits(self, qapp):
+        """Test that the Delimiters page has 4 line edits."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        line_edits = ui.page_delimiters.findChildren(QtWidgets.QLineEdit)
+        assert len(line_edits) == 4, "Delimiters page should have 4 line edits"
+
+    def test_all_delimiter_line_edits_accessible(self, qapp):
+        """Test that all delimiter line edits are accessible."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        line_edit_names = [
+            "le_metadata_delimiter_artist",
+            "le_metadata_delimiter_album_artist",
+            "le_filename_delimiter_artist",
+            "le_filename_delimiter_album_artist",
+        ]
+
+        for name in line_edit_names:
+            line_edit = getattr(ui, name, None)
+            assert line_edit is not None, f"LineEdit {name} not found"
+            assert isinstance(line_edit, QtWidgets.QLineEdit)
+
+    def test_delimiter_line_edits_have_max_width(self, qapp):
+        """Test that delimiter line edits have a maximum width set."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        line_edit_names = [
+            "le_metadata_delimiter_artist",
+            "le_metadata_delimiter_album_artist",
+            "le_filename_delimiter_artist",
+            "le_filename_delimiter_album_artist",
+        ]
+
+        for name in line_edit_names:
+            line_edit = getattr(ui, name)
+            # Should have max width of 100px
+            assert line_edit.maximumWidth() == 100, f"{name} should have max width of 100px"
+
+    def test_delimiter_labels_exist(self, qapp):
+        """Test that labels and icon labels exist for each delimiter."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        label_names = [
+            ("l_metadata_delimiter_artist", "l_icon_metadata_delimiter_artist"),
+            ("l_metadata_delimiter_album_artist", "l_icon_metadata_delimiter_album_artist"),
+            ("l_filename_delimiter_artist", "l_icon_filename_delimiter_artist"),
+            ("l_filename_delimiter_album_artist", "l_icon_filename_delimiter_album_artist"),
+        ]
+
+        for label_name, icon_label_name in label_names:
+            label = getattr(ui, label_name, None)
+            icon_label = getattr(ui, icon_label_name, None)
+
+            assert label is not None, f"Label {label_name} not found"
+            assert icon_label is not None, f"Icon label {icon_label_name} not found"
+            assert isinstance(label, QtWidgets.QLabel)
+            assert isinstance(icon_label, QtWidgets.QLabel)
+
+
+class TestDelimitersIntegration:
+    """Test Delimiters integration with DialogPreferences."""
+
+    @pytest.fixture
+    def mock_settings(self):
+        """Create a mock Settings object with delimiter attributes."""
+        settings = Mock()
+        settings.data = Mock()
+
+        # Boolean flags
+        for attr in [
+            "lyrics_embed",
+            "lyrics_file",
+            "use_primary_album_artist",
+            "video_download",
+            "download_dolby_atmos",
+            "download_delay",
+            "video_convert_mp4",
+            "extract_flac",
+            "metadata_cover_embed",
+            "mark_explicit",
+            "cover_album_file",
+            "skip_existing",
+            "symlink_to_track",
+            "playlist_create",
+        ]:
+            setattr(settings.data, attr, False)
+
+        # Paths
+        for attr in [
+            "download_base_path",
+            "format_album",
+            "format_playlist",
+            "format_mix",
+            "format_track",
+            "format_video",
+            "path_binary_ffmpeg",
+        ]:
+            setattr(settings.data, attr, f"/test/{attr}")
+
+        # Delimiters
+        settings.data.metadata_delimiter_artist = ", "
+        settings.data.metadata_delimiter_album_artist = ", "
+        settings.data.filename_delimiter_artist = ", "
+        settings.data.filename_delimiter_album_artist = ", "
+
+        # Numbers
+        settings.data.album_track_num_pad_min = 2
+        settings.data.downloads_concurrent_max = 3
+
+        # Enums
+        settings.data.quality_audio = Mock()
+        settings.data.quality_video = Mock()
+        settings.data.metadata_cover_dimension = Mock()
+
+        return settings
+
+    @patch.object(DialogPreferences, "exec")
+    def test_delimiters_category_added_to_list(self, mock_exec, mock_settings, qapp):
+        """Test that Delimiters category is added to the list."""
+        from PySide6.QtCore import Signal as QtSignal
+
+        settings_save = QtSignal()
+
+        with patch.object(DialogPreferences, "gui_populate"):
+            dialog = DialogPreferences(mock_settings, settings_save)
+
+            # Should have 5 categories now
+            assert dialog.ui.lw_categories.count() == 5
+            assert dialog.ui.lw_categories.item(4).text() == "Delimiters"
+
+    @patch.object(DialogPreferences, "exec")
+    def test_delimiter_line_edits_in_parameters(self, mock_exec, mock_settings, qapp):
+        """Test that delimiter line edits are in parameters_line_edit."""
+        from PySide6.QtCore import Signal as QtSignal
+
+        settings_save = QtSignal()
+
+        with patch.object(DialogPreferences, "gui_populate"):
+            dialog = DialogPreferences(mock_settings, settings_save)
+
+            # Check that all 4 delimiters are in the list
+            assert "metadata_delimiter_artist" in dialog.parameters_line_edit
+            assert "metadata_delimiter_album_artist" in dialog.parameters_line_edit
+            assert "filename_delimiter_artist" in dialog.parameters_line_edit
+            assert "filename_delimiter_album_artist" in dialog.parameters_line_edit
+
+    @patch.object(DialogPreferences, "exec")
+    def test_navigate_to_delimiters_page(self, mock_exec, mock_settings, qapp):
+        """Test navigation to Delimiters page."""
+        from PySide6.QtCore import Signal as QtSignal
+
+        settings_save = QtSignal()
+
+        with patch.object(DialogPreferences, "gui_populate"):
+            dialog = DialogPreferences(mock_settings, settings_save)
+
+            # Navigate to Delimiters (index 4)
+            dialog.ui.lw_categories.setCurrentRow(4)
+            assert dialog.ui.sw_categories.currentIndex() == 4
+            assert dialog.ui.sw_categories.currentWidget() == dialog.ui.page_delimiters
+
+
+class TestDelimitersStructure:
+    """Test the structural integrity of the Delimiters page."""
+
+    def test_stacked_widget_has_five_pages(self, qapp):
+        """Test that the stacked widget now contains 5 pages."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        assert ui.sw_categories.count() == 5, "Should have 5 pages including Delimiters"
+
+    def test_delimiters_page_has_layout(self, qapp):
+        """Test that Delimiters page has a proper layout."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        assert ui.page_delimiters.layout() is not None
+        assert isinstance(ui.page_delimiters.layout(), QtWidgets.QVBoxLayout)
+
+    def test_delimiters_page_has_spacer(self, qapp):
+        """Test that Delimiters page has a vertical spacer."""
+        dialog = QtWidgets.QDialog()
+        ui = Ui_DialogSettings()
+        ui.setupUi(dialog)
+
+        # Check that there's a spacer in the layout
+        layout = ui.page_delimiters.layout()
+        spacer_found = False
+        for i in range(layout.count()):
+            item = layout.itemAt(i)
+            if item.spacerItem():
+                spacer_found = True
+                break
+
+        assert spacer_found, "Delimiters page should have a vertical spacer"