refactor: apply review cuts

- replace hand-rolled clamp_cursor_to_char_boundary with
  str::floor_char_boundary at its four call sites, drop the helper
- dedupe the duplicated Layout::vertical constraint arms in render()
- drop stale #[allow(dead_code)] on spinner_state (field is read)

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

@@ -21,7 +21,6 @@ pub struct MainTui {
     #[allow(dead_code)]
     max_concurrent_downloads: usize,
     search_timeout: Duration,
-    #[allow(dead_code)]
     spinner_state: usize,
 }