Add tag cloud feature with k-means clustering and CUDA support

Introduces an Explore view with a tag cloud that clusters image embeddings
using cosine k-means and labels clusters via vocabulary-nearest-neighbour
CLIP matching. Vocabulary embeddings are disk-cached (FNV hash-keyed) to
avoid redundant inference. Enables CUDA for candle dependencies and adds a
build.rs check that surfaces a clear error when the toolkit is missing.

src-tauri/src/lib.rs

@@ -77,6 +77,7 @@ pub fn run() {
             commands::semantic_search_images,
             commands::set_worker_paused,
             commands::get_worker_states,
+            commands::get_tag_cloud,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");