Add municipal receipt module and convert navbar to sidebar

- New module: Municipalidad de Belén receipt extraction via pdftotext+regex
  - Backend: MunicipalReceipt + WaterMeterReading models, upload/list/detail/water-consumption endpoints
  - Auto-creates budget Transaction on upload (duplicate-safe via reference)
  - Frontend: ServiciosMunicipales page with summary cards, water consumption bar chart, receipt history, PDF upload
- Convert top navbar to left sidebar with section headers (General, Finanzas, Servicios)
  - Desktop: fixed 220px sidebar, mobile: sheet overlay
  - Grouped nav: Dashboard | Presupuesto, Salarios, Pensiones, Analytics | Municipalidad

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

frontend/src/api.ts

@@ -304,3 +304,73 @@ export const getPensionFundSummary = () =>
 
 export const submitPensionManualEntries = (entries: PensionManualEntry[]) =>
   api.post<PensionUploadResult>('/pensions/manual', { entries });
+
+// --- Municipal Receipts ---
+
+export interface MunicipalCharge {
+  detail: string;
+  amount: number;
+}
+
+export interface WaterMeterReading {
+  id: number;
+  meter_id: string;
+  period: string;
+  reading_previous: number;
+  reading_current: number;
+  consumption_m3: number;
+  agua_potable: number;
+  serv_ambientales: number;
+  alcant_sanitario: number;
+  iva: number;
+  is_historical: boolean;
+  receipt_id: number | null;
+  created_at: string;
+}
+
+export interface MunicipalReceipt {
+  id: number;
+  receipt_date: string;
+  due_date: string;
+  period: string;
+  account: string;
+  finca: string;
+  holder_name: string;
+  holder_cedula: string;
+  holder_address: string;
+  subtotal: number;
+  interests: number;
+  iva: number;
+  total: number;
+  raw_charges: MunicipalCharge[];
+  source_filename: string;
+  created_at: string;
+}
+
+export interface MunicipalReceiptDetail extends MunicipalReceipt {
+  water_readings: WaterMeterReading[];
+}
+
+export interface MunicipalReceiptUploadResult {
+  imported: number;
+  updated: number;
+  errors: string[];
+  receipt: MunicipalReceipt | null;
+}
+
+export const uploadMunicipalReceipt = (file: File) => {
+  const form = new FormData();
+  form.append('file', file);
+  return api.post<MunicipalReceiptUploadResult>('/municipal-receipts/upload', form);
+};
+
+export const getMunicipalReceipts = () =>
+  api.get<MunicipalReceipt[]>('/municipal-receipts/');
+
+export const getMunicipalReceiptDetail = (id: number) =>
+  api.get<MunicipalReceiptDetail>(`/municipal-receipts/${id}`);
+
+export const getWaterConsumption = (months?: number) =>
+  api.get<WaterMeterReading[]>('/municipal-receipts/water-consumption', {
+    params: months ? { months } : undefined,
+  });