Add cumulative balance tracking with editable overrides

- New BalanceOverride table for manual balance adjustments per month
- Cumulative balance computation with cross-year carryover
- Three new columns: Acum. Anterior, Neto Mes, Balance Acum.
- Inline editing on Balance Acum. cell (pencil icon for overrides)
- Year navigation clamped to 2026–2030, fresh start at March 2026
- PUT/DELETE /budget/balance-override/{year}/{month} endpoints

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/hooks/useBudget.ts

@@ -11,6 +11,8 @@ import {
   createRecurringItem,
   updateRecurringItem as apiUpdateItem,
   deleteRecurringItem as apiDeleteItem,
+  upsertBalanceOverride,
+  deleteBalanceOverride,
 } from '@/api';
 
 export function useBudget(initialYear: number) {
@@ -71,6 +73,16 @@ export function useBudget(initialYear: number) {
     await Promise.all([fetchProjection(), fetchMonthDetail(), fetchRecurringItems()]);
   };
 
+  const saveBalanceOverride = async (overrideYear: number, month: number, value: number) => {
+    await upsertBalanceOverride(overrideYear, month, value);
+    await fetchProjection();
+  };
+
+  const clearBalanceOverride = async (overrideYear: number, month: number) => {
+    await deleteBalanceOverride(overrideYear, month);
+    await fetchProjection();
+  };
+
   return {
     year,
     setYear,
@@ -84,6 +96,8 @@ export function useBudget(initialYear: number) {
     addItem,
     updateItem,
     deleteItem,
+    saveBalanceOverride,
+    clearBalanceOverride,
     refresh: () => Promise.all([fetchProjection(), fetchMonthDetail(), fetchRecurringItems()]),
   };
 }