feat(discovery): surface all tags and guard folder removal

Include tags used by a single image in Explore and invalidate the tag cache after edits so changes appear immediately. Add an accessible, time-limited confirmation step before removing indexed folders from the sidebar.

src-tauri/src/db.rs

@@ -1523,7 +1523,7 @@ pub fn get_explore_tags(
          JOIN images i ON i.id = t.image_id
          WHERE (?1 IS NULL OR i.folder_id = ?1)
          GROUP BY t.tag
-         HAVING COUNT(DISTINCT t.image_id) >= 2
+         HAVING COUNT(DISTINCT t.image_id) >= 1
          ORDER BY tag_count DESC, t.tag ASC
          LIMIT ?2",
     )?;