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/pages/Budget.tsx

@@ -18,8 +18,11 @@ const MONTH_NAMES = [
   'Julio', 'Agosto', 'Septiembre', 'Octubre', 'Noviembre', 'Diciembre',
 ];
 
+const MIN_YEAR = 2026;
+const MAX_YEAR = 2030;
+
 export default function Budget() {
-  const currentYear = new Date().getFullYear();
+  const currentYear = Math.max(MIN_YEAR, new Date().getFullYear());
   const {
     year,
     setYear,
@@ -33,6 +36,8 @@ export default function Budget() {
     addItem,
     updateItem,
     deleteItem,
+    saveBalanceOverride,
+    clearBalanceOverride,
     refresh,
   } = useBudget(currentYear);
 
@@ -81,11 +86,11 @@ export default function Budget() {
           <h1 className="text-2xl font-bold tracking-tight">Presupuesto</h1>
         </div>
         <div className="flex items-center gap-1">
-          <Button variant="outline" size="icon" onClick={() => setYear(year - 1)}>
+          <Button variant="outline" size="icon" disabled={year <= MIN_YEAR} onClick={() => setYear(year - 1)}>
             <ChevronLeft className="w-4 h-4" />
           </Button>
           <span className="w-16 text-center font-semibold tabular-nums">{year}</span>
-          <Button variant="outline" size="icon" onClick={() => setYear(year + 1)}>
+          <Button variant="outline" size="icon" disabled={year >= MAX_YEAR} onClick={() => setYear(year + 1)}>
             <ChevronRight className="w-4 h-4" />
           </Button>
         </div>
@@ -194,10 +199,17 @@ export default function Budget() {
                     <YearlyOverview
                       months={projection.months}
                       selectedMonth={selectedMonth}
+                      year={year}
                       onSelectMonth={(m) => {
                         setSelectedMonth(m);
                         setSubTab('detail');
                       }}
+                      onSaveOverride={async (month, value) => {
+                        await saveBalanceOverride(year, month, value);
+                      }}
+                      onClearOverride={async (month) => {
+                        await clearBalanceOverride(year, month);
+                      }}
                     />
                   </CardContent>
                 </Card>