feat(tui): show the clear-all key in the searches pane's shortcuts

C has cleared every search for a while, and the keys popup says so, but
the pane's own shortcut bar never mentioned it, so it read as if d on
each search were the only way. The bar now lists it next to d → remove.

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

@@ -594,6 +594,7 @@ impl MainTui {
                     ("s", "search"),
                     ("Enter", "results"),
                     ("d", "remove"),
+                    ("C", "clear all"),
                     ("m", "message"),
                     ("i", "inbox"),
                     ("c", "chat"),