test: add data-testid to mode buttons, admin button, and quantity controls

frontend/app/page.tsx

@@ -592,6 +592,7 @@ export default function Home() {
             ].map((m) => (
               <button
                 key={m.id}
+                data-testid={m.id === 'CHECK_IN' ? 'operation-checkin' : m.id === 'CHECK_OUT' ? 'operation-checkout' : undefined}
                 onClick={() => setMode(m.id as any)}
                 className={cn(
                   "flex-1 py-3.5 rounded-xl text-xs sm:text-sm font-black transition-all flex items-center justify-center gap-3",
@@ -872,7 +873,7 @@ export default function Home() {
                       >
                         <Minus size={24} />
                       </button>
-                      <div className="text-center">
+                      <div className="text-center" data-testid="adjustment-quantity-input">
                         <span className="text-xs font-black tabular-nums">{adjustQty}</span>
                         <span className="text-[10px] text-primary/80 font-bold tracking-tight">Units</span>
                       </div>

frontend/components/BottomNav.tsx

@@ -60,6 +60,7 @@ export default function BottomNav({
         {/* Admin Settings */}
         {currentUser?.role === 'admin' && (
           <button
+            data-testid="admin-button"
             onClick={() => router.push('/admin')}
             aria-label="Go to Admin"
             className={cn("flex flex-col items-center gap-1 rounded px-2 py-1 cursor-pointer transition-colors focus:ring-2 focus:ring-primary focus:outline-none", isAdmin && "text-primary")}