feat: related-tags atlas in Explore view

Hovering a tag in Explore now loads and displays co-occurring tags as a
weighted cloud. New `get_related_tags` SQL self-join (db.rs/commands.rs),
`loadRelatedTags` store action with per-folder keyed cache, and TagCloud
atlas UI with ResizeObserver-driven layout and RAF animation. Explore tag
limit raised to 180; tag cloud auto-refreshes 700ms after new AI tagging
completes.

src-tauri/src/lib.rs

@@ -197,6 +197,7 @@ pub fn run() {
             commands::get_worker_states,
             commands::get_tag_cloud,
             commands::get_explore_tags,
+            commands::get_related_tags,
             commands::get_images_by_ids,
             commands::get_failed_embedding_images,
             commands::get_failed_tagging_images,