feat: freeze frame at playhead

Enable the timeline toolbar freeze-frame button: capture the composited
canvas as PNG, add it as an image media asset, split the video element
at the playhead, shift the right part by the freeze duration, and insert
a 3-second image element in the gap. Adds RendererManager.captureFrame(),
reused by createSnapshot().

Also adds the Nix devshell (flake.nix/flake.lock: bun + nodejs) and
AGENTS.md repository guidelines.

1 files changed, 27 insertions, 0 deletions

diff --git a/flake.lock b/flake.lock
new file mode 100644
index 00000000..0b9301e2
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1784796856,
+        "narHash": "sha256-wWFrV5/Qbm+lyt5x20E/bSbfJiGKMo4RCxZV8cl/WZI=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e2587caef70cea85dd97d7daab492899902dbf5d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}