Surface failed embeddings and add filter for affected files

- Fix root cause: embedding_source_path() returns Result<PathBuf>, returning
  Err for videos without a thumbnail instead of silently falling back to the
  raw .mp4 path that CLIP cannot decode
- indexer: split embedding batch into pre-failed (no source) and embeddable
  jobs; pre-failed are marked immediately without hitting the CLIP model
- db: retry_failed_embedding_jobs skips videos still without a thumbnail so
  they no longer re-fail immediately on retry
- Add get_failed_embedding_images command listing failed files + error per folder
- Gallery: amber warning badge on tiles with embedding_status = 'failed'
- BackgroundTasks: fetch and show failed filenames/errors in expanded panel
- Toolbar: conditional 'Failed Embeddings' amber filter pill shown when any
  folder has embedding_failed > 0; filters at DB level via new
  embedding_failed_only param on get_images / count_images
- TagCloud: replaced vocabulary/dictionary label system with representative
  image thumbnails per cluster; results cached in SQLite by image-id hash

src-tauri/src/lib.rs

@@ -78,6 +78,7 @@ pub fn run() {
             commands::set_worker_paused,
             commands::get_worker_states,
             commands::get_tag_cloud,
+            commands::get_failed_embedding_images,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");