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/settings/page.tsx

@@ -119,7 +119,7 @@ export default function SettingsPage() {
         </h2>
         {stats ? (
           <>
-            <div className="grid grid-cols-3 gap-4">
+            <div className="grid grid-cols-1 sm:grid-cols-3 gap-4">
               <div className="bg-muted/50 rounded-lg p-3 text-center">
                 <FileText className="w-4 h-4 mx-auto mb-1 text-muted-foreground" />
                 <div className="text-xl font-bold">{stats.total_bills.toLocaleString()}</div>