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/components/AssetSidebar.vue

@@ -2,6 +2,7 @@
 import { computed, ref, watch } from 'vue'
 import {
   PhCircle,
+  PhCursorClick,
   PhFilmStrip,
   PhImage,
   PhImageSquare,
@@ -107,6 +108,7 @@ async function addSampleText() {
 }
 
 const clips = computed(() => store.scene.clips ?? [])
+const buttonClips = computed(() => store.scene.buttonClips ?? [])
 
 async function addClip() {
   const input = await promptDialog({
@@ -147,6 +149,45 @@ function onClipDragStart(e: DragEvent, clipId: string) {
   e.dataTransfer.setData('text/plain', clipId)
 }
 
+async function addButtonClip() {
+  const input = await promptDialog({
+    title: t('sidebar.newButtonDialog.title'),
+    message: t('sidebar.newButtonDialog.message'),
+    defaultValue: t('sidebar.newButtonDialog.default'),
+    confirmLabel: t('sidebar.newButtonDialog.confirm'),
+  })
+  if (input === null) return
+  store.seedDefaultButtonClip(input || t('sidebar.newButtonDialog.default'))
+}
+
+async function renameButtonClipPrompt(id: string, current: string) {
+  const input = await promptDialog({
+    title: t('sidebar.renameButtonDialog.title'),
+    defaultValue: current,
+    confirmLabel: t('sidebar.renameButtonDialog.confirm'),
+  })
+  if (input === null) return
+  store.setButtonClipName(id, input)
+}
+
+async function deleteButtonClip(id: string) {
+  const ok = await confirmDialog({
+    title: t('sidebar.deleteButtonDialog.title'),
+    message: t('sidebar.deleteButtonDialog.message'),
+    confirmLabel: t('sidebar.deleteButtonDialog.confirm'),
+    variant: 'danger',
+  })
+  if (!ok) return
+  store.removeButtonClip(id)
+}
+
+function onButtonClipDragStart(e: DragEvent, id: string) {
+  if (!e.dataTransfer) return
+  e.dataTransfer.effectAllowed = 'copy'
+  e.dataTransfer.setData('application/x-animatr-button-clip', id)
+  e.dataTransfer.setData('text/plain', id)
+}
+
 async function handleFileSelected(event: Event) {
   uploadError.value = null
   const input = event.target as HTMLInputElement
@@ -336,6 +377,51 @@ function onAssetDragStart(e: DragEvent, asset: Extract<Asset, { kind: 'image' }>
       </li>
       <li v-if="!clips.length" class="empty">{{ t('sidebar.noClips') }}</li>
     </ul>
+
+    <div class="lib-header clips-header">
+      <h4>{{ t('sidebar.buttons') }}</h4>
+      <button
+        class="thumb-toggle"
+        :title="t('sidebar.createButtonTooltip')"
+        @click="addButtonClip"
+      >
+        <PhPlus :size="12" weight="bold" />
+      </button>
+    </div>
+    <ul class="clips">
+      <li
+        v-for="bc in buttonClips"
+        :key="bc.id"
+        class="clip button-clip"
+        draggable="true"
+        @dragstart="onButtonClipDragStart($event, bc.id)"
+      >
+        <div class="clip-icon">
+          <PhCursorClick :size="16" />
+        </div>
+        <div class="clip-meta">
+          <span class="clip-name" :title="bc.name">{{ bc.name }}</span>
+          <span class="clip-sub">{{ t('sidebar.buttonSubtitle') }}</span>
+        </div>
+        <div class="clip-actions">
+          <button
+            class="clip-btn"
+            :title="t('sidebar.renameTooltip')"
+            @click.stop="renameButtonClipPrompt(bc.id, bc.name)"
+          >
+            <PhPencilSimple :size="11" />
+          </button>
+          <button
+            class="clip-btn danger"
+            :title="t('sidebar.deleteTooltip')"
+            @click.stop="deleteButtonClip(bc.id)"
+          >
+            <PhTrash :size="11" />
+          </button>
+        </div>
+      </li>
+      <li v-if="!buttonClips.length" class="empty">{{ t('sidebar.noButtons') }}</li>
+    </ul>
   </div>
 </template>