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>

server/server.js

@@ -103,10 +103,19 @@ function effectiveEnd(scene) {
   return Math.min(max, scene.duration)
 }
 
+function playbackRange(scene) {
+  const autoEnd = effectiveEnd(scene)
+  const start = Math.max(0, Math.min(scene.duration, scene.inTime == null ? 0 : scene.inTime))
+  const rawEnd = scene.outTime == null ? autoEnd : scene.outTime
+  const end = Math.max(start, Math.min(scene.duration, rawEnd))
+  return { start, end }
+}
+
 async function runJob(job, scene, options) {
   const fps = Number(options.fps || scene.fps || 60)
-  const sceneEnd = effectiveEnd(scene)
-  const duration = Number(options.duration || sceneEnd)
+  const { start: rangeIn, end: rangeOut } = playbackRange(scene)
+  const rangeLen = Math.max(0, rangeOut - rangeIn)
+  const duration = Number(options.duration || rangeLen)
   const totalFrames = Math.max(1, Math.round(duration * fps))
   const crf = String(options.crf ?? 18)
   const preset = String(options.preset ?? 'fast')
@@ -162,7 +171,7 @@ async function runJob(job, scene, options) {
         try { ffmpeg.kill('SIGKILL') } catch { /* noop */ }
         throw new Error('aborted')
       }
-      const t = Math.min(i / fps, duration)
+      const t = rangeIn + Math.min(i / fps, duration)
       await page.evaluate((time) => window.__animatrSeek(time), t)
       const buf = await page.screenshot({
         type: 'png',