Hide editor picker dialog on mobile

Skip showing the EditorChooserModal when the viewport is narrower than
640px (the sm breakpoint), since the two-column picker layout is not
suited for mobile screens.

https://claude.ai/code/session_01RPriMsYb7BnWdbB4CnnSuW

src/routes/edit/+page.svelte

@@ -55,7 +55,7 @@
   let showEditorChooser = $state(false);
 
   onMount(async () => {
-    showEditorChooser = shouldShowEditorChooser();
+    showEditorChooser = shouldShowEditorChooser() && window.innerWidth >= 640;
     await initHandler();
     window.addEventListener('appinstalled', () => {
       logEvent('pwaInstalled', { isMobile });