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/indexer.rs

@@ -1288,7 +1288,7 @@ fn max_worker_fetch_size(active_folders: &HashSet<i64>) -> usize {
         .unwrap_or(StorageProfile::Balanced.worker_fetch_size())
 }
 
-fn with_db_write_lock<T>(operation: impl FnOnce() -> Result<T>) -> Result<T> {
+pub fn with_db_write_lock<T>(operation: impl FnOnce() -> Result<T>) -> Result<T> {
     let lock = DB_WRITE_LOCK.get_or_init(|| Mutex::new(()));
     let _guard = lock.lock().unwrap();
     operation()