feat(tui): clear the whole queue from the transfers pane

Shift-C in the Downloads/Uploads pane now cancels every live download,
forgets every row — queued, finished or failed — in the session too,
and cancels uploads still streaming, so a shared queue empties for
every window at once. The existing c keeps clearing only finished
transfers.

soulseek-rs/src/ui/main_tui/input.rs

@@ -569,6 +569,9 @@ impl MainTui {
             KeyCode::Char('c') => {
                 self.clear_finished_downloads();
             }
+            KeyCode::Char('C') => {
+                self.clear_all_transfers();
+            }
             _ => {}
         }
     }