feat(tui): tabbed browsing — browse several users at once

Replace the single browse view with BrowseTabs (one tab per user),
mirroring the chat-room tabs. Browsing a new user (from search results
or a chat room's member list) opens or focuses their tab; Tab/Shift-Tab
switch between open users, 'w' closes the active tab, Esc hides the
popup while keeping the tabs. A tab bar shows the open users when more
than one is open, and each tab polls/loads independently. Re-browsing a
timed-out user retries. BrowseTabs is unit-tested.

soulseek-rs/src/models/mod.rs

@@ -7,6 +7,8 @@ pub use app_state::{
     AppState, ChatMessage, CommandBarMode, DownloadEntry, FocusedPane,
     MessageDirection, SearchEntry, SearchStatus,
 };
-pub use browse::{BrowseState, BrowseStatus, files_under, find_node};
+pub use browse::{
+    BrowseState, BrowseStatus, BrowseTabs, files_under, find_node,
+};
 pub use file_display_data::FileDisplayData;
 pub use rooms::{RoomLine, RoomsState, RoomsView};