extend ClideTappable with press/longpress, retrofit button + rail

ClideTappable builder now receives (context, hovered, pressed).
Optional onLongPress and onPressChanged callbacks. ClideButton
is now a StatelessWidget wrapping ClideTappable. ClideIconRail
refactored — _RailButton uses ClideTappable with tooltip.
ClideTooltip positions smart (above when near screen bottom).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/builtin/welcome/src/welcome_view.dart

@@ -307,7 +307,7 @@ class _ThemeLink extends StatelessWidget {
   Widget build(BuildContext context) {
     return ClideTappable(
       onTap: () => kernel.commands.execute('theme.pick'),
-      builder: (ctx, hovered) => Row(
+      builder: (ctx, hovered, _) => Row(
         mainAxisSize: MainAxisSize.min,
         children: [
           ClideText('theme: ', muted: true, fontSize: 12, fontFamily: clideMonoFamily),