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/components/PropertiesPanel.vue

@@ -593,21 +593,21 @@ function removeKeyframeHere() {
               <button
                 type="button"
                 class="icon-toggle"
-                :class="{ active: (selected.maskMode ?? 'positive') === 'positive' }"
-                :title="t('properties.maskPositive')"
-                @click="store.setLayerMaskMode(selected.id, 'positive')"
-              >
-                <PhPlus :size="13" weight="bold" />
-              </button>
-              <button
-                type="button"
-                class="icon-toggle"
-                :class="{ active: selected.maskMode === 'negative', danger: selected.maskMode === 'negative' }"
+                :class="{ active: (selected.maskMode ?? 'negative') === 'negative', danger: (selected.maskMode ?? 'negative') === 'negative' }"
                 :title="t('properties.maskNegative')"
                 @click="store.setLayerMaskMode(selected.id, 'negative')"
               >
                 <PhMinus :size="13" weight="bold" />
               </button>
+              <button
+                type="button"
+                class="icon-toggle"
+                :class="{ active: selected.maskMode === 'positive' }"
+                :title="t('properties.maskPositive')"
+                @click="store.setLayerMaskMode(selected.id, 'positive')"
+              >
+                <PhPlus :size="13" weight="bold" />
+              </button>
             </div>
           </div>
           <p v-if="selected.isMask && maskInGroup" class="status-active">