fix: resolve all dart analyze warnings

- Migrate RadioListTile to RadioGroup API (Flutter 3.32+)
- Guard BuildContext usage across async gaps
- Use const BorderRadius.all / EdgeInsets.all constructors
- Extract nested ternaries into helpers
- Prefer .first over [0], ??= over if-null assignment
- Prefix unused FocusNode params with _
- Add library directive for dangling doc comments

lib/widgets/overlay_sheet.dart

@@ -392,7 +392,7 @@ class _OverlaySheetHostState extends State<OverlaySheetHost> with SingleTickerPr
     }
   }
 
-  KeyEventResult _handleKeyEvent(FocusNode node, KeyEvent event) {
+  KeyEventResult _handleKeyEvent(FocusNode _, KeyEvent event) {
     // Suppress stale select key-ups
     if (SelectKeyUpSuppressor.consumeIfSuppressed(event)) {
       return KeyEventResult.handled;