Add monthly summary and annual overview (Phase 4)

Monthly summary:
- GET /api/summary/monthly returns income, bills, actuals, one-time
  expense totals and surplus/deficit for a given month
- MonthlySummary page shows stat cards and breakdown table

Annual overview:
- AnnualOverview page fetches all 12 months in parallel via Promise.all
- Year navigation, summary cards, monthly table with totals row
- Fix: normalize nested API response to flat fields expected by component

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/pages/AnnualOverview.jsx

@@ -37,12 +37,25 @@ export default function AnnualOverview() {
     setLoading(true);
     setError(null);
 
+    // Normalize nested API response to flat fields used by this component
+    function normalize(data) {
+      if (!data) return null;
+      return {
+        total_income: data.income?.net ?? 0,
+        total_bills: data.bills?.planned ?? 0,
+        total_variable: data.actuals?.total ?? 0,
+        total_one_time: data.one_time_expenses?.total ?? 0,
+        total_spending: data.summary?.total_spending ?? 0,
+        surplus_deficit: data.summary?.surplus_deficit ?? 0,
+      };
+    }
+
     Promise.all(
       Array.from({ length: 12 }, (_, i) =>
         fetch(`/api/summary/monthly?year=${year}&month=${i + 1}`)
           .then(r => {
             if (!r.ok) return null;
-            return r.json();
+            return r.json().then(normalize);
           })
           .catch(() => null)
       )