feat: add landing page statistics page with recharts

Add AdminLandingStats page with:
- Summary cards (purchases, revenue, gifts, conversion)
- AreaChart with dual Y-axis (daily purchases + revenue)
- BarChart (tariff distribution)
- PieChart donut (gift vs regular breakdown)
- Responsive layout for desktop and mobile
- Stats button on landing cards list

Includes API types, route, translations (ru/en).

src/api/landings.ts

@@ -274,6 +274,37 @@ export const landingApi = {
   },
 };
 
+// ============================================================
+// Admin stats types
+// ============================================================
+
+export interface LandingDailyStat {
+  date: string;
+  purchases: number;
+  revenue_kopeks: number;
+  gifts: number;
+}
+
+export interface LandingTariffStat {
+  tariff_id: number | null;
+  tariff_name: string;
+  purchases: number;
+  revenue_kopeks: number;
+}
+
+export interface LandingStatsResponse {
+  total_purchases: number;
+  total_revenue_kopeks: number;
+  total_gifts: number;
+  total_regular: number;
+  avg_purchase_kopeks: number;
+  total_created: number;
+  total_successful: number;
+  conversion_rate: number;
+  daily_stats: LandingDailyStat[];
+  tariff_stats: LandingTariffStat[];
+}
+
 // ============================================================
 // Admin API
 // ============================================================
@@ -312,4 +343,9 @@ export const adminLandingsApi = {
   reorder: async (landingIds: number[]): Promise<void> => {
     await apiClient.put('/cabinet/admin/landings/order', { landing_ids: landingIds });
   },
+
+  getStats: async (id: number): Promise<LandingStatsResponse> => {
+    const response = await apiClient.get(`/cabinet/admin/landings/${id}/stats`);
+    return response.data;
+  },
 };