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/members/page.tsx

@@ -56,7 +56,7 @@ export default function MembersPage() {
       ) : (
         <>
           <div className="text-sm text-muted-foreground">{data?.total ?? 0} members</div>
-          <div className="grid grid-cols-2 gap-3">
+          <div className="grid grid-cols-1 sm:grid-cols-2 gap-3">
             {data?.items?.map((member) => (
               <div key={member.bioguide_id} className="bg-card border border-border rounded-lg p-4 flex items-start justify-between gap-3">
                 <div className="flex-1 min-w-0">