macro-clips slice 4c: button creation in sidebar + drag to clip tracks

- store.seedDefaultButtonClip(name) adds a button clip with a single
  shared rect asset and four pre-populated state stacks in blue shades
  (normal/hover/pressed/disabled). Layer name 'Face' is consistent so
  transitions can later pair layers by name.
- AssetSidebar gets a Buttons section below Clips: create via prompt,
  rename, delete, drag-to-track. Drag uses a new dataTransfer type
  'application/x-animatr-button-clip' to keep it distinct from
  animation-clip drops.
- Timeline clip-lane drop accepts both types and dispatches
  addClipInstance / addButtonInstance accordingly; the new-track
  drop zone handles both too, so dragging a button to an empty area
  creates a fresh clip track + instance in one step.
- i18n for the sidebar strings and three dialogs.
- test covers seedDefaultButtonClip: four populated states, shared
  asset, consistent layer name.

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

src/i18n/messages.ts

@@ -95,6 +95,25 @@ export const en = {
       message: 'The clip and all placed instances will be removed.',
       confirm: 'Delete',
     },
+    buttons: 'Buttons',
+    buttonSubtitle: 'Button · 4 states',
+    noButtons: 'No buttons yet',
+    createButtonTooltip: 'Create new button',
+    newButtonDialog: {
+      title: 'New button',
+      message: 'Button name:',
+      default: 'Button',
+      confirm: 'Create',
+    },
+    renameButtonDialog: {
+      title: 'Rename button',
+      confirm: 'Apply',
+    },
+    deleteButtonDialog: {
+      title: 'Delete button?',
+      message: 'The button and all placed instances will be removed.',
+      confirm: 'Delete',
+    },
   },
 
   properties: {