0.6.0: background video, in/out range, top-level masks, track icons

- background video layer (document-level), syncs to playhead in editor and fullscreen
- playback in/out brackets on the scrub ruler; i/o set them, r rewinds to in-point
- togglePlay, tick, fullscreen loop, WebM + server FFmpeg export all honor [in, out]
- top-level mask layers cascade over everything below until a group boundary
- fullscreen preview slaves localTime to the bg video so it stops stuttering
- welcome "new project" now actually loads a blank scene (was a no-op)
- default canvas background hardened to white via normalizeScene
- new keyframe button (+) in the name-spacer, backspace/delete removes selected kfs
- drag-end auto-selects the created/shifted keyframe so the path is visible
- animation path updates live during drag and spans every selected keyframe
- track rows show an icon per asset kind (rect/circle/image/text/polygon/freeform)
- default mask mode is negative, buttons read − then +
- sticky track-name columns raised above playhead/brackets; orange bracket styling
- tests for playback range, blank scene, mask defaults, cascading top-level masks

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/styles/global.css

@@ -13,6 +13,8 @@
   --accent-soft-strong: rgba(110, 168, 255, 0.22);
   --danger: #ff7a7a;
   --danger-soft: rgba(255, 122, 122, 0.12);
+  --range-bracket: #f59e0b;
+  --range-hover: #ffffff;
   --radius: 6px;
   --radius-sm: 4px;
   --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.2);
@@ -36,6 +38,8 @@
   --accent-soft-strong: rgba(43, 106, 245, 0.18);
   --danger: #d63c3c;
   --danger-soft: rgba(214, 60, 60, 0.09);
+  --range-bracket: #d97706;
+  --range-hover: #111111;
   --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.06);
   --shadow-md: 0 8px 24px rgba(20, 25, 40, 0.12);
   color-scheme: light;