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-tauri/src/lib.rs

@@ -192,6 +192,7 @@ pub fn run() {
             commands::open_app_data_folder,
             commands::get_database_info,
             commands::vacuum_database,
+            commands::rebuild_semantic_index,
             commands::get_orphaned_thumbnails_info,
             commands::cleanup_orphaned_thumbnails,
             commands::get_muted_folder_ids,