refactor(analytics): Simplify analytics endpoints and eliminate client-side processing.

- Add /transactions/monthly-stats endpoint with SQL aggregation
- Replace client-side monthly processing with server-side calculations
- Reduce data transfer by 99.5% (2,507 → 13 records for yearly data)
- Simplify MonthlyTrends component by removing 40+ lines of aggregation logic
- Clean up unused imports and interfaces

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -60,7 +60,7 @@ export const apiClient = {
     const queryParams = new URLSearchParams();
     if (days) queryParams.append("days", days.toString());
     if (accountId) queryParams.append("account_id", accountId);
-    
+
     const response = await api.get<ApiResponse<Balance[]>>(
       `/balances/history?${queryParams.toString()}`
     );
@@ -161,7 +161,7 @@ export const apiClient = {
   getTransactionStats: async (days?: number): Promise<TransactionStats> => {
     const queryParams = new URLSearchParams();
     if (days) queryParams.append("days", days.toString());
-    
+
     const response = await api.get<ApiResponse<TransactionStats>>(
       `/transactions/stats?${queryParams.toString()}`
     );
@@ -172,12 +172,33 @@ export const apiClient = {
   getTransactionsForAnalytics: async (days?: number): Promise<AnalyticsTransaction[]> => {
     const queryParams = new URLSearchParams();
     if (days) queryParams.append("days", days.toString());
-    
+
     const response = await api.get<ApiResponse<AnalyticsTransaction[]>>(
       `/transactions/analytics?${queryParams.toString()}`
     );
     return response.data.data;
   },
+
+  // Get monthly transaction statistics (pre-calculated)
+  getMonthlyTransactionStats: async (days?: number): Promise<Array<{
+    month: string;
+    income: number;
+    expenses: number;
+    net: number;
+  }>> => {
+    const queryParams = new URLSearchParams();
+    if (days) queryParams.append("days", days.toString());
+
+    const response = await api.get<ApiResponse<Array<{
+      month: string;
+      income: number;
+      expenses: number;
+      net: number;
+    }>>>(
+      `/transactions/monthly-stats?${queryParams.toString()}`
+    );
+    return response.data.data;
+  },
 };
 
 export default apiClient;