feat: mobile-optimize admin section

- Layout: sidebar hidden on mobile, replaced with horizontal top nav strip
- Tables: overflow-x-auto + hide secondary columns (email/category/dates/
  plays/likes) on small screens; show email inline under name on mobile
- Forms: grid-cols-2 → grid-cols-1 sm:grid-cols-2 on all admin forms
- Markdown editor: Write/Preview tab toggle on mobile, side-by-side on sm+
- Padding: p-3 sm:p-6 on all admin pages for tighter mobile layout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/admin/videos/[id]/+page.svelte

@@ -88,7 +88,7 @@
   }
 </script>
 
-<div class="p-6 max-w-2xl">
+<div class="p-3 sm:p-6 max-w-2xl">
   <div class="flex items-center gap-4 mb-6">
     <Button variant="ghost" href="/admin/videos" size="sm">
       <span class="icon-[ri--arrow-left-line] h-4 w-4 mr-1"></span>Back
@@ -97,7 +97,7 @@
   </div>
 
   <div class="space-y-5">
-    <div class="grid grid-cols-2 gap-4">
+    <div class="grid grid-cols-1 sm:grid-cols-2 gap-4">
       <div class="space-y-1.5">
         <Label for="title">Title *</Label>
         <Input id="title" bind:value={title} placeholder="Video title" />