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/bills/[id]/page.tsx

@@ -78,8 +78,8 @@ export default function BillDetailPage({ params }: { params: Promise<{ id: strin
       </div>
 
       {/* Content grid */}
-      <div className="grid grid-cols-3 gap-6">
-        <div className="col-span-2 space-y-6">
+      <div className="grid grid-cols-1 md:grid-cols-3 gap-4 md:gap-6">
+        <div className="md:col-span-2 space-y-6">
           <AIBriefCard brief={bill.latest_brief} />
           <ActionTimeline actions={bill.actions} />
         </div>