feat: 5-frame button assets + timeline/sidebar polish

Button assets
- Re-introduce button-asset type with 4 content states (normal/hover/
  pressed/disabled) stored as keyframes at BUTTON_STATE_TIMES, plus a
  separate hit-area editor mode.
- useButtonRuntime composable for per-instance state/disabled flags on
  the main stage.
- Sidebar library lists button assets with live 80×80 thumbnails;
  renderButtonThumbnail now accepts an images map so image-kind layers
  inside a button render instead of producing a fully-transparent PNG.
- Compact (no-thumbnail) mode aligns button rows to the library-row
  height.

Scope footer
- Button-edit mode now uses the same sidebar scope-footer pattern as
  clip-edit (back button to "Main timeline" + active scope chip);
  removed the inline exit button from the timeline scrub-bar.

Timeline
- Track-name column default bumped to 260px to match the assets sidebar
  above it (storage key bumped to :v2 so stale values reset).
- scrub-bar z-index raised above sticky track-name columns so rows
  scroll under it instead of over.
- scrub-handle flips z-index based on horizontal scroll: above
  name-spacer when the playhead is in view (no half-clip at t=0),
  below when the playhead has scrolled off the left edge (so the
  toggle row cleanly covers it).
- name-spacer extends ±3/1px to fully cover the obscured handle.
- scrub-handle head pokes 3px above the ruler with rounded top corners
  via clip-path path().

Track context menu
- Added missing i18n keys (renameScriptTrack / deleteScriptTrack /
  renameScriptTrackDialog.{title,confirm}).
- Rename items show a visible pencil icon; empty .check slot removed
  so items line up flush-left; toggle items place their active ✓ on
  the right (kf-menu-trailing-check).

Dev HUD
- Test progress bar flips to --danger as soon as the first failure is
  recorded, instead of only at the end of the run.

Regression tests
- renderButtonThumbnail: null on empty button, renders shape layers,
  accepts custom images map.
- i18n: renameScriptTrack / deleteScriptTrack / dialog keys resolve.
- scrub-handle-obscured logic (visible at t=0, hidden when playhead is
  scrolled off-screen, still visible when playhead is far to the
  right of the scroll offset).
- Updated button state-scrubber expectations to match
  BUTTON_STATE_TIMES (normal=1, hover=2, pressed=3, disabled=4) and
  that hit-mode parks at normal=1 rather than 0.

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

src/App.vue

@@ -4,6 +4,7 @@ import {
   PhArrowClockwise,
   PhArrowCounterClockwise,
   PhArrowLeft,
+  PhCursorClick,
   PhExport,
   PhFilmStrip,
   PhFolderOpen,
@@ -439,6 +440,16 @@ onBeforeUnmount(() => {
           {{ store.activeClip?.name ?? '' }}
         </span>
       </div>
+      <div v-else-if="store.activeButtonAssetId" class="scope-footer">
+        <button class="scope-back" @click="store.exitButtonAssetEdit">
+          <PhArrowLeft :size="12" weight="bold" />
+          {{ t('app.mainTimeline') }}
+        </button>
+        <span class="scope-current" :title="store.activeButtonAsset?.name ?? ''">
+          <PhCursorClick :size="12" />
+          {{ store.activeButtonAsset?.name ?? '' }}
+        </span>
+      </div>
     </aside>
     <main class="stage"><CanvasStage /></main>
     <aside class="properties"><PropertiesPanel /></aside>