macro-clips slice 4b: button instance dialog

- ButtonInstanceDialog.vue edits the three instance-scope fields:
  label (with uniqueness error surface), disabled flag, onClick
  actions via the shared ActionEditor
- commit waits on a successful label set so collisions show inline
  instead of silently dropping the rename
- Timeline wires click on a button-instance block to open the dialog

Creation/placement UX (sidebar entry + drag-to-track) and the per-state
layer editor are still deferred — this slice only makes existing
instances editable once something (devtools, later slice) has put them
on a clip track.

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

src/i18n/messages.ts

@@ -396,6 +396,17 @@ export const en = {
     remove: 'Delete frame',
     done: 'Done',
   },
+  buttonInstanceDialog: {
+    title: 'Button instance',
+    clipName: 'Button',
+    label: 'Label',
+    labelPlaceholder: 'e.g. playButton',
+    labelCollision: 'Label "{name}" is already used by another button instance.',
+    disabled: 'Start disabled',
+    onClick: 'On click',
+    remove: 'Delete instance',
+    done: 'Done',
+  },
 } as const
 
 export type Messages = typeof en