fix(tui): make browse retry work and de-jargon the copy

- Retry actually retries: 'r' in the browse popup re-requests a
  timed-out user (previously the 'Press b to retry' hint did nothing —
  b wasn't handled inside the popup). BrowseTabs::retry_active is
  unit-tested.
- Friendlier timeout copy that explains the likely cause: "Couldn't
  reach <user>. They may be offline, or their connection can't be
  reached (both of you may be behind a router/firewall). Press r to try
  again."
- Replace protocol terminology in user-facing labels: 'Peer slots' ->
  'Free slots', 'Peer upload speed' -> 'Upload speed'.

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

@@ -104,12 +104,12 @@ fn build_info_lines(
                 } else {
                     "all busy".to_string()
                 };
-                lines.push(label_value("Peer slots", &slot_text));
+                lines.push(label_value("Free slots", &slot_text));
             }
 
             if let Some(speed) = download.metadata.peer_upload_speed {
                 lines.push(label_value(
-                    "Peer upload speed",
+                    "Upload speed",
                     &format_speed(f64::from(speed)),
                 ));
             }