refactor: convert promocode stats modal to separate page

- Create AdminPromocodeStats.tsx with responsive mobile layout
- Add route /admin/promocodes/:id/stats
- Fix mobile layout in AdminPromocodes.tsx (badges and buttons no longer overlap)
- Remove PromocodeStatsModal component

src/App.tsx

@@ -42,6 +42,7 @@ const AdminBroadcasts = lazy(() => import('./pages/AdminBroadcasts'));
 const AdminBroadcastCreate = lazy(() => import('./pages/AdminBroadcastCreate'));
 const AdminPromocodes = lazy(() => import('./pages/AdminPromocodes'));
 const AdminPromocodeCreate = lazy(() => import('./pages/AdminPromocodeCreate'));
+const AdminPromocodeStats = lazy(() => import('./pages/AdminPromocodeStats'));
 const AdminPromoGroups = lazy(() => import('./pages/AdminPromoGroups'));
 const AdminPromoGroupCreate = lazy(() => import('./pages/AdminPromoGroupCreate'));
 const AdminCampaigns = lazy(() => import('./pages/AdminCampaigns'));
@@ -393,6 +394,16 @@ function App() {
             </AdminRoute>
           }
         />
+        <Route
+          path="/admin/promocodes/:id/stats"
+          element={
+            <AdminRoute>
+              <LazyPage>
+                <AdminPromocodeStats />
+              </LazyPage>
+            </AdminRoute>
+          }
+        />
         <Route
           path="/admin/promo-groups"
           element={