feat: browse the highlighted transfer's user with b

In the Downloads / Uploads pane, b now opens the share of whoever owns
the selected download or upload, the way it already browses the owner of
a highlighted result. With no row selected it still prompts.

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"),
+            ("b", "browse that user"),
         ],
     ),
     (