Add Restocking Orders feature: view, API, and Orders integration

[DineshvsAccenture]

• Add Restocking.vue: new page for browsing and submitting restocking orders; lets warehouse staff select low-stock items and submit a consolidated order with budget validation
• Register /restocking route in main.js and add nav link in App.vue
• api.js: add getRestockingOrders() and createRestockingOrder() helpers
• server/main.py: add RestockingOrder/RestockingItem Pydantic models; POST /api/restocking-orders (201, 7-day fixed lead time, sequential order number RST-YYYY-NNNN); GET /api/restocking-orders
• server/mock_data.py: initialise in-memory restocking_orders list (resets on restart, consistent with the no-persistence demo pattern)
• Orders.vue: show submitted restocking orders in a summary card below the main orders table; errors fetching supplemental data are logged but do not block the main orders view
• Add architecture.html: high-level system diagram