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/components/shared/MobileHeader.tsx

@@ -0,0 +1,16 @@
+"use client";
+import { Menu, Landmark } from "lucide-react";
+
+export function MobileHeader({ onMenuClick }: { onMenuClick: () => void }) {
+  return (
+    <header className="md:hidden flex items-center justify-between px-4 py-3 border-b border-border bg-card shrink-0">
+      <button onClick={onMenuClick} className="p-2 rounded-md hover:bg-accent transition-colors" aria-label="Open menu">
+        <Menu className="w-5 h-5" />
+      </button>
+      <div className="flex items-center gap-2">
+        <Landmark className="w-5 h-5 text-primary" />
+        <span className="font-semibold text-sm">PocketVeto</span>
+      </div>
+    </header>
+  );
+}