Add version control to track editor, restyle story list

- Story items can be pinned to a specific generation version via
  toolbar dropdown (shows when clip is selected and has >1 version)
- version_id column on story_items with migration, validated against
  the generation's versions before saving
- Split/duplicate preserve the source clip's pinned version
- Export and playback resolve version-specific audio paths
- Extracted _build_item_detail helper in stories.py (DRY cleanup)
- Story list restyled from rounded cards to flat rows with rounded
  hover/active states, gradient header fade, and dynamic bottom
  padding that accounts for track editor + generate box

app/src/lib/api/client.ts

@@ -27,6 +27,7 @@ import type {
   StoryItemReorder,
   StoryItemSplit,
   StoryItemTrim,
+  StoryItemVersionUpdate,
   StoryResponse,
   TranscriptionResponse,
   VoiceProfileCreate,
@@ -588,6 +589,17 @@ class ApiClient {
     });
   }
 
+  async setStoryItemVersion(
+    storyId: string,
+    itemId: string,
+    data: StoryItemVersionUpdate,
+  ): Promise<StoryItemDetail> {
+    return this.request<StoryItemDetail>(`/stories/${storyId}/items/${itemId}/version`, {
+      method: 'PUT',
+      body: JSON.stringify(data),
+    });
+  }
+
   async exportStoryAudio(storyId: string): Promise<Blob> {
     const url = `${this.getBaseUrl()}/stories/${storyId}/export-audio`;
     const response = await fetch(url);

app/src/lib/api/types.ts

@@ -222,6 +222,7 @@ export interface StoryItemDetail {
   id: string;
   story_id: string;
   generation_id: string;
+  version_id?: string;
   start_time_ms: number;
   track: number;
   trim_start_ms: number;
@@ -236,6 +237,12 @@ export interface StoryItemDetail {
   seed?: number;
   instruct?: string;
   generation_created_at: string;
+  versions?: GenerationVersionResponse[];
+  active_version_id?: string;
+}
+
+export interface StoryItemVersionUpdate {
+  version_id: string | null;
 }
 
 export interface StoryDetailResponse {