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

src/ui/views/calendar.rs

@@ -1,11 +1,11 @@
 //! Calendar view - upcoming releases.
 
 use ratatui::{
+    Frame,
     layout::{Constraint, Layout, Rect},
     style::{Modifier, Style},
     text::{Line, Span},
     widgets::Paragraph,
-    Frame,
 };
 
 use crate::data::CalendarEntry;