refactor: replace vim keybindings with mouse navigation, remove mock data

- Remove all sample/mock data (artists, albums, tracks, queue, history, calendar)
- Delete input module (vim.rs, leader.rs) and related UI (which_key, cmdline)
- Add mouse support: click tabs, click list items, scroll wheel navigation
- Show real disk space via nix::statvfs instead of hardcoded value
- Simplify topbar/statusbar by removing mode display and key hints
- Hide album/track sections when no artist is selected

flake.nix

@@ -60,6 +60,8 @@
               rust-analyzer
               clippy
               rustfmt
+
+              opencode
             ];
           };
         };