0.8.4: button transitions, locked groups revoke edits, UX polish

Button-state transitions
- State flips on stage-preview buttons (normal / hover / pressed /
  disabled) now ease over 50ms instead of snapping. New
  `transitions` map + rAF ticker in useButtonRuntime drive a
  continuous sample time between BUTTON_STATE_TIMES slots;
  CanvasStage samples each button child at that interpolated time
  so keyframes at the 4 states blend smoothly. Mid-flight flips
  (hover → pressed while still in normal → hover) rebase from
  the current live sample instead of snapping.

Locked groups revoke edits
- Locking a layer directly or its enclosing group deselects the
  layer so the Properties panel can't keep editing it. Panel also
  splits into three branches — editable / locked / no-layer —
  with a dedicated locked-layer hint.
- Timeline keyframe interactions (drag, dblclick-to-remove,
  context menu, dblclick-to-create on lane, add-kf toolbar)
  now consult effective lock (direct + parent group) instead
  of just layer.locked.

Editor polish
- Inactive tracks fade as a whole row (opacity 0.5) instead of
  per-child, so keyframes, segments, and still-frame tails stay
  visually proportional to their active counterparts.
- Dragging a layer onto a group's track-name label now drops
  it INTO the group (was previously only the group-lane area).
- Button assets on the stage show a pointer cursor on hover so
  it's obvious they're interactive.
- Global CSS forces `cursor: pointer !important` on every enabled
  button / role="button" so scoped component styles can no longer
  accidentally shadow the default.
- Timeline resize handle jumped above the scrub-bar (z-index 100)
  so the full 14px grab zone is hit-testable again.

Regression tests
- setTrackLocked deselect paths: direct lock, group lock with
  selected child, group lock with unrelated selection, unlock
  doesn't re-select.
- duplicateKeyframeAt: props + easing + hold + bezier copied,
  collision refusal, unknown-id guards, separated-track routing.
- Button transition runtime: no-transition baseline, mid-flight
  bounds, mid-transition rebase, no-op setState.
- Shared composables tooltips + asset-thumbs: same ref across
  calls, localStorage round-trip.

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

src/i18n/messages.ts

@@ -139,6 +139,8 @@ export const en = {
     removeKeyframe: 'Remove keyframe here',
     deleteLayer: 'Delete layer',
     noLayer: 'No layer selected.',
+    lockedLayer:
+      'Selected layer is locked. Unlock the track (or its group) to edit properties.',
     fill: 'Fill',
     transparent: 'Transparent',
     opacity: 'Opacity',