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>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "animatr",
   "private": true,
-  "version": "0.5.6",
+  "version": "0.6.0",
   "type": "module",
   "scripts": {
     "dev": "vite",