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/render.rs

@@ -89,6 +89,7 @@ const HELP_RIGHT: &[(&str, &[(&str, &str)])] = &[
             ("r", "retry a failed one"),
             ("d", "delete a queued or finished one"),
             ("c", "clear every finished one"),
+            ("C", "clear every one, cancelling the live ones"),
             ("b", "browse that user"),
         ],
     ),
@@ -702,6 +703,7 @@ impl MainTui {
                     ("r", "retry failed"),
                     ("d", "delete queued/done"),
                     ("c", "clear finished"),
+                    ("C", "clear all"),
                     ("b", "browse user"),
                     ("h/l", "scroll name"),
                 ],