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/topics/page.tsx

@@ -37,7 +37,7 @@ export default function TopicsPage() {
         </p>
       </div>
 
-      <div className="grid grid-cols-2 gap-3">
+      <div className="grid grid-cols-1 sm:grid-cols-2 gap-3">
         {TOPICS.map(({ tag, label, desc }) => (
           <div key={tag} className="bg-card border border-border rounded-lg p-4 flex items-start justify-between gap-3">
             <div>