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, 128 insertions, 0 deletions

diff --git a/docs/refactor_settings_ui.md b/docs/refactor_settings_ui.md
new file mode 100644
index 0000000..651427c
--- /dev/null
+++ b/docs/refactor_settings_ui.md
@@ -0,0 +1,128 @@
+# Settings Window Refactor
+
+Date: 2025-11-30
+Author: Automation (Copilot)
+
+## Goal
+
+Refactor the settings window to improve usability and maintainability:
+
+- Category list on the left
+- Settings content on the right via QStackedWidget
+- Clear separation of pages (Flags, Quality, Numbers, Paths & Formats, Delimiters)
+- Consistent dark styling and tuned spacing
+- Add unit and integration tests (61 total)
+
+## Key Changes
+
+### 1. UI Architecture
+
+- Navigation column: `QListWidget (lw_categories)`
+- Pages container: `QStackedWidget (sw_categories)`
+- Pages created:
+  - `page_flags` (GroupBox: gb_flags)
+  - `page_quality` (GroupBox: gb_choices)
+  - `page_numbers` (GroupBox: gb_numbers)
+  - `page_paths` (GroupBox: gb_path)
+  - `page_delimiters` (GroupBox: gb_delimiters) — NEW
+
+Impacted file: `tidal_dl_ng/ui/dialog_settings.ui` (auto-generated `dialog_settings.py`).
+
+### 2. New Category: Delimiters
+
+- "Delimiters" page with 4 `QLineEdit` fields to edit separators:
+  - `metadata_delimiter_artist`
+  - `metadata_delimiter_album_artist`
+  - `filename_delimiter_artist`
+  - `filename_delimiter_album_artist`
+- Automatic integration with `DialogPreferences` through `parameters_line_edit`, `populate_line_edit()` and `to_settings()`.
+
+Files touched:
+
+- `tidal_dl_ng/ui/dialog_settings.ui` (new page)
+- `tidal_dl_ng/ui/dialog_settings.py` (regenerated)
+- `tidal_dl_ng/dialog.py` (added 4 parameters and new category in `_init_categories`).
+
+### 3. Styles & Spacing
+
+- Category list: dark style, blue selection, gray hover, light text.
+- Vertical spacing: list item margin `2px 0px` (and optional view spacing).
+- Settings area: dark background (QStackedWidget), styled GroupBoxes.
+
+Files touched:
+
+- `tidal_dl_ng/ui/dialog_settings.ui` (stylesheets on lw_categories and sw_categories)
+- `tidal_dl_ng/ui/dialog_settings.py` (regenerated).
+
+### 4. Tests
+
+- New files:
+  - `tests/test_settings_ui.py`: UI and basic integration (14)
+  - `tests/test_settings_dialog_structure.py`: detailed structure (20)
+  - `tests/test_dialog_preferences_integration.py`: DialogPreferences integration (14)
+  - `tests/test_delimiters_category.py`: Delimiters page (13)
+- Total: **61 tests**; ruff/black/pyupgrade applied via pre-commit.
+
+### 5. Pre-commit & Quality
+
+- Ruff fixes:
+  - F841 (unused variable) in `test_dialog_preferences_integration.py`
+  - S108 (using /tmp) replaced by `Path.home()` and safe values
+- end-of-file-fixer and black reformatted automatically.
+
+## Usage
+
+### Launch the UI
+
+- Via the app: `poetry run tidal-dl-ng-gui` (or `python -m tidal_dl_ng.gui`)
+
+### Navigate
+
+- Select a category in `lw_categories`
+- The corresponding page shows in `sw_categories`.
+
+### Edit separators
+
+- Open the "Delimiters" category
+- Edit fields `le_metadata_delimiter_*` and `le_filename_delimiter_*`
+- Confirm via OK
+
+## Known issue: poetry+dulwich KeyError b'HEAD
+
+- Cause: dulwich cannot resolve HEAD in the Git repo when Poetry queries VCS info.
+- Fix: ensure `.git/HEAD` points to an existing branch (e.g. `ref: refs/heads/main`) and the branch exists; then re-run `poetry install`. See commands below.
+
+## Useful Commands (PowerShell)
+
+```powershell
+cd C:\Users\mathe\PycharmProjects\tidal-dl-ng
+poetry check --lock
+poetry run pre-commit run -a
+pytest -q
+```
+
+To fix HEAD if needed:
+
+```powershell
+cd C:\Users\mathe\PycharmProjects\tidal-dl-ng
+Get-Content .git\HEAD
+# If empty/incorrect:
+git init
+git add .
+git commit -m "Initialize repo for Poetry/Dulwich"
+git branch -M main
+Get-Content .git\HEAD
+poetry install -vvv --no-interaction --all-extras --with dev,docs
+```
+
+## Impact
+
+- Better UX: category navigation, improved readability
+- Extensible: easy to add new pages/settings
+- Quality: comprehensive tests ensure robustness
+
+## Next Steps
+
+- Add contextual help (tooltips) for new delimiter fields
+- Optionally adjust list spacing (8–10) based on feedback
+- Add a preview of resulting formatting (e.g., sample metadata)