fix(board): fit preview to panel, keep app selection on search, keep checklist under modals

- Scale oversized widget previews to fit the 320px panel
- Derive selected apps from all apps so search doesn't drop selections
- Drop checklist Affix z-index override so modals stay on top
- Always render preview panel (placeholder state) and add custom widget previews
- Hide preview column below lg breakpoint to avoid crushing the grid

packages/modals-collection/src/apps/app-select-modal.tsx

@@ -34,10 +34,7 @@ export const AppSelectModal = createModal<AppSelectModalProps>(({ actions, inner
     [apps, search],
   );
 
-  const selectedApps = useMemo(
-    () => filteredApps.filter((app) => selectedAppIds.has(app.id)),
-    [filteredApps, selectedAppIds],
-  );
+  const selectedApps = useMemo(() => apps.filter((app) => selectedAppIds.has(app.id)), [apps, selectedAppIds]);
 
   const handleSelect = (app: SelectableApp) => {
     if (multiSelect) {