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/api.ts

@@ -186,6 +186,9 @@ export interface MonthlyProjection {
   uncovered_actual: number;
   gran_total_egresos: number;
   net_balance: number;
+  carryover_balance: number;
+  cumulative_balance: number;
+  balance_overridden: boolean;
 }
 
 export interface YearlyProjection {
@@ -225,6 +228,10 @@ export const getYearlyProjection = (year: number) =>
   api.get<YearlyProjection>(`/budget/projection/${year}`);
 export const getMonthlyDetail = (year: number, month: number) =>
   api.get<MonthlyDetail>(`/budget/month/${year}/${month}`);
+export const upsertBalanceOverride = (year: number, month: number, override_balance: number) =>
+  api.put(`/budget/balance-override/${year}/${month}`, { override_balance });
+export const deleteBalanceOverride = (year: number, month: number) =>
+  api.delete(`/budget/balance-override/${year}/${month}`);
 
 // --- Salarios ---