feat(tui): browse or PM a user from a chat room's member list

The member list in an open room is now selectable with the arrow keys.
Press 'b' to browse the highlighted user's shared files (opens the
browse tree) or 'm' to send them a private message (opens the compose
bar pre-filled with the recipient). Selection resets when switching
rooms; RoomsState navigation is unit-tested.

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

@@ -132,7 +132,7 @@ fn render_chat(frame: &mut Frame, area: Rect, rooms: &RoomsState) {
             .split(chunks[1]);
 
     render_messages(frame, body[0], active.lines.as_slice());
-    render_users(frame, body[1], &active.users);
+    render_users(frame, body[1], &active.users, rooms.user_selected);
 
     // Compose line or hint.
     if rooms.composing {
@@ -200,7 +200,12 @@ fn render_messages(
     frame.render_widget(Paragraph::new(rendered), area);
 }
 
-fn render_users(frame: &mut Frame, area: Rect, users: &[String]) {
+fn render_users(
+    frame: &mut Frame,
+    area: Rect,
+    users: &[String],
+    selected: usize,
+) {
     let block = Block::default()
         .borders(Borders::LEFT)
         .border_style(dimmed_style())
@@ -209,10 +214,20 @@ fn render_users(frame: &mut Frame, area: Rect, users: &[String]) {
     frame.render_widget(block, area);
 
     let height = inner.height as usize;
+    // Keep the highlighted user in view by scrolling the window to it.
+    let start = selected.saturating_sub(height.saturating_sub(1));
     let shown: Vec<Line> = users
         .iter()
+        .enumerate()
+        .skip(start)
         .take(height)
-        .map(|u| Line::from(Span::styled(u.clone(), primary_style())))
+        .map(|(i, u)| {
+            if i == selected {
+                Line::from(Span::styled(format!("▸ {u}"), highlight_style()))
+            } else {
+                Line::from(Span::styled(format!("  {u}"), primary_style()))
+            }
+        })
         .collect();
     frame.render_widget(Paragraph::new(shown), inner);
 }