feat: overall overtime balance on history page

Backend:
- ClosedWeekStore.SumDelta: single SQL aggregate returning total delta_ms and
  row count across all closed_weeks
- WeekService.Balance: thin passthrough returning BalanceResult{TotalDeltaMs, ClosedWeekCount}
- GET /api/weeks/balance handler; route registered alongside /weeks list/close/reopen
- Tests: store-level SumDelta (empty + populated), service-level Balance (empty + 2 weeks)

Frontend:
- weeks.balance() added to API client
- History page: balance card at top, fetched in parallel with existing data
- Loading state shows '—'; once loaded shows formatDelta value in green/red/gray
- Shows 'across N closed weeks' count alongside the value

web/src/lib/api/client.ts

@@ -124,7 +124,8 @@ export const weeks = {
 		return request<ClosedWeek[]>('GET', `/weeks?${params}`);
 	},
 	close: (weekKey: string) => request<ClosedWeek>('POST', `/weeks/${weekKey}/close`),
-	reopen: (weekKey: string) => request<void>('DELETE', `/weeks/${weekKey}/close`)
+	reopen: (weekKey: string) => request<void>('DELETE', `/weeks/${weekKey}/close`),
+	balance: () => request<{ total_delta_ms: number; closed_week_count: number }>('GET', '/weeks/balance')
 };
 
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