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/models/browse.rs

@@ -371,6 +371,20 @@ impl BrowseTabs {
         }
     }
 
+    /// Retry the active tab if it timed out: reset it to loading and return the
+    /// username so the caller can re-issue the request. Returns `None` if the
+    /// active tab isn't in a retryable (timed-out) state.
+    pub fn retry_active(&mut self) -> Option<String> {
+        let tab = self.tabs.get_mut(self.active)?;
+        if tab.status == BrowseStatus::TimedOut {
+            let username = tab.username.clone();
+            *tab = BrowseState::loading(username.clone());
+            Some(username)
+        } else {
+            None
+        }
+    }
+
     /// Close the active tab, returning `false` when no tabs remain.
     pub fn close_active(&mut self) -> bool {
         if self.active < self.tabs.len() {
@@ -545,6 +559,20 @@ mod tests {
         assert_eq!(tabs.active_tab().unwrap().status, BrowseStatus::Loading);
     }
 
+    #[test]
+    fn browse_tabs_retry_active_only_when_timed_out() {
+        let mut tabs = BrowseTabs::new();
+        tabs.open("alice");
+        // Loading tab isn't retryable.
+        assert_eq!(tabs.retry_active(), None);
+        tabs.active_tab_mut().unwrap().status = BrowseStatus::TimedOut;
+        // Timed-out tab retries and resets to loading.
+        assert_eq!(tabs.retry_active().as_deref(), Some("alice"));
+        assert_eq!(tabs.active_tab().unwrap().status, BrowseStatus::Loading);
+        // Now loading again, no further retry.
+        assert_eq!(tabs.retry_active(), None);
+    }
+
     #[test]
     fn browse_state_load_auto_expands_top_level() {
         let mut state = BrowseState::loading("bob".to_string());