Split mouse.rs into page-specific handler files

Extract handle_artists_click and handle_playlists_click into
mouse_artists.rs and mouse_playlists.rs respectively, reducing
mouse.rs from ~530 to ~247 lines.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -9,6 +9,8 @@ mod input_queue;
 mod input_server;
 mod input_settings;
 mod mouse;
+mod mouse_artists;
+mod mouse_playlists;
 mod playback;
 pub mod state;