feat(tui): chat rooms — browsable list, multiple open rooms, unread badges

Add a chat-rooms popup (open with 'c') satisfying the requested UX:
- Browsable, /-filterable room list showing per-room user counts,
  busiest first; already-open rooms are marked.
- Multiple rooms open at once as tabs; Tab/Shift-Tab switch, 'x' leaves.
- Per-room chat log with a live member list and an inline compose line
  (Enter to type, Enter to send; the server echo renders the message).
- Unread indicators: bold tab name + 'room (n)' badge per room, plus
  'chat (n)' and 'inbox (n)' counters in the shortcut bar; viewing a
  room clears its badge.

RoomsState is unit-tested (routing, unread accounting, tab lifecycle,
filtering).

soulseek-rs/src/ui/panes/mod.rs

@@ -2,10 +2,12 @@ mod browse_pane;
 mod download_info_pane;
 mod downloads_pane;
 mod results_pane;
+mod rooms_pane;
 mod searches_pane;
 
 pub use browse_pane::render_browse_pane;
 pub use download_info_pane::render_download_info_pane;
 pub use downloads_pane::render_downloads_pane;
 pub use results_pane::{ResultsPaneParams, render_results_pane};
+pub use rooms_pane::render_rooms_pane;
 pub use searches_pane::render_searches_pane;