feat: Fase 1-3 completas - precios proveedor, multi-sucursal, factura global

Fase 1: Lista de precios de proveedor
- Tabla supplier_catalog_prices en master DB
- Endpoints GET/POST/PUT/DELETE /supplier-catalog/prices
- Upload CSV/Excel de precios de proveedor
- Visualizacion de supplier_price en catalogo y POS

Fase 2: Multi-sucursal completo
- Migracion v4.0: inventory.branch_id=NULL, tabla inventory_stock
- Campos fiscales en branches (RFC, regimen, CP, serie CFDI, certificados)
- Trigger trg_update_inventory_stock para sincronizar stock por sucursal
- Backend config_bp.py con CRUD de sucursales fiscales
- Backend inventory_bp.py y pos_bp.py refactorizados para inventario compartido
- Backend invoicing_bp.py usa datos fiscales de la sucursal de la venta
- Frontend config.html/js con modal de sucursales expandido

Fase 3: Factura global mensual
- Migracion v4.1: tablas global_invoice_sales, sales.global_invoiced_at
- build_global_invoice_xml() con InformacionGlobal SAT-compliant
- Servicio global_invoice.py para agrupar ventas PUE <=000
- Endpoints POST/GET /global-invoice y /global-invoice/eligible-sales
- Frontend invoicing.html/js con boton y modal de factura global

pos/templates/pos.html

@@ -206,6 +206,7 @@
 
         <!-- Secondary Actions -->
         <div class="secondary-actions" role="toolbar" aria-label="Acciones secundarias">
+          <button class="btn-secondary-action" onclick="POS.modifyPrice()" title="Modificar precio">Mod.Precio</button>
           <button class="btn-secondary-action" onclick="POS.saveQuotation()" title="Cotizacion (F4)">Cotizar</button>
           <button class="btn-secondary-action" onclick="POS.showLastSale()" title="Ultima venta (F5)">Ult.Venta</button>
           <button class="btn-secondary-action" onclick="POS.showCutZModal()" title="Corte Z - Cerrar caja">Corte Z</button>
@@ -570,7 +571,7 @@
   <script src="/pos/static/js/splash-loader.js?v=1" defer></script>
   <script src="/pos/static/js/push.js" defer></script>
   <script src="/pos/static/js/printer.js" defer></script>
-  <script src="/pos/static/js/pos.js?v=6" defer></script>
+  <script src="/pos/static/js/pos.js?v=7" defer></script>
 
   <script>
     // Cancel sale button wiring