feat(settings): add "Rebuild semantic index" maintenance action

Drops and recreates the sqlite-vec tables at the current model dimension, then
re-queues every image for embedding. Fixes "dimension mismatch" search errors
that occur when the vector table was built for a different model/dimension (e.g.
after experimenting with 768-dim models against this 512-dim build).

The rebuild runs under the embedding worker's DB write lock and resets the job
queue inside a single transaction, so it can't interleave with an in-flight
embedding batch (per code review).

src/store.ts

@@ -467,6 +467,7 @@ interface GalleryState {
   openAppDataFolder: () => Promise<void>;
   getDatabaseInfo: () => Promise<DatabaseInfo>;
   vacuumDatabase: () => Promise<VacuumResult>;
+  rebuildSemanticIndex: () => Promise<number>;
   getOrphanedThumbnailsInfo: () => Promise<OrphanedThumbnailsInfo>;
   cleanupOrphanedThumbnails: () => Promise<CleanupOrphanedThumbnailsResult>;
   retryFailedEmbeddings: (folderId: number) => Promise<void>;
@@ -1858,6 +1859,8 @@ export const useGalleryStore = create<GalleryState>((set, get) => ({
 
   vacuumDatabase: () => invoke<VacuumResult>("vacuum_database"),
 
+  rebuildSemanticIndex: () => invoke<number>("rebuild_semantic_index"),
+
   getOrphanedThumbnailsInfo: () => invoke<OrphanedThumbnailsInfo>("get_orphaned_thumbnails_info"),
 
   cleanupOrphanedThumbnails: () => invoke<CleanupOrphanedThumbnailsResult>("cleanup_orphaned_thumbnails"),