Regenerate as new version, UI polish, and bugfixes

- Add /generate/{id}/regenerate endpoint that creates a new take version
- Wire regenerate into history dropdown with SSE progress + autoplay
- Add normalize_audio to regenerate path
- Remove duplicate Regenerate menu item
- Show disabled ellipsis menu during generation instead of hiding it
- Fix sf.write format kwarg broken by asyncio.to_thread migration
- Rename clean version label to 'original', effects to 'version-N'
- Show effects chain names in version list instead of 'N fx'
- Include all versions in export package
- Clean up version panel padding

app/src/lib/api/client.ts

@@ -212,6 +212,12 @@ class ApiClient {
     });
   }
 
+  async regenerateGeneration(generationId: string): Promise<GenerationResponse> {
+    return this.request<GenerationResponse>(`/generate/${generationId}/regenerate`, {
+      method: 'POST',
+    });
+  }
+
   // History
   async listHistory(query?: HistoryQuery): Promise<HistoryListResponse> {
     const params = new URLSearchParams();