feat(ui): add mobile-responsive layout with hamburger drawer

Adds MobileHeader with hamburger button (left-aligned) that opens a
slide-in sidebar drawer on mobile. Desktop layout is unchanged. All
hardcoded multi-column grids updated with responsive Tailwind breakpoints.

Co-Authored-By: Jack Levy

frontend/app/page.tsx

@@ -40,8 +40,8 @@ export default function DashboardPage() {
       {isLoading ? (
         <div className="text-center py-20 text-muted-foreground">Loading dashboard...</div>
       ) : (
-        <div className="grid grid-cols-3 gap-8">
-          <div className="col-span-2 space-y-4">
+        <div className="grid grid-cols-1 md:grid-cols-3 gap-4 md:gap-8">
+          <div className="md:col-span-2 space-y-4">
             <h2 className="font-semibold flex items-center gap-2">
               <BookOpen className="w-4 h-4" />
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