feat: basic POS & invoicing (closes groombook/groombook#5) (#26)

- Add invoice_status and payment_method enums to schema
- Add invoices table: appointmentId, clientId, subtotal/tax/tip/total cents,
  status (draft/pending/paid/void), paymentMethod, paidAt, notes
- Add invoice_line_items table: invoiceId, description, qty, unitPrice, total
- Migration 0002_invoices.sql with FK constraints and journal entry
- POST /api/invoices — create invoice with line items
- POST /api/invoices/from-appointment/:id — one-click invoice from appointment,
  pre-populated with service name and price; returns 409 if already invoiced
- GET /api/invoices — list with optional ?status/clientId/appointmentId filters
- GET /api/invoices/:id — invoice with line items
- PATCH /api/invoices/:id — update status, payment method, tip, notes; auto-sets
  paidAt when marking paid; blocks edits on voided invoices
- Add Invoice/InvoiceLineItem types to @groombook/types
- InvoicesPage: list view with status filter, create from appointment modal,
  detail modal with tip input, payment method selector, Mark as Paid/Void actions
- Add Invoices nav link in App.tsx

Co-authored-by: Groom Book CTO <cto@groombook.app>
Co-authored-by: Paperclip <noreply@paperclip.ing>

apps/web/src/App.tsx

@@ -3,12 +3,14 @@ import { AppointmentsPage } from "./pages/Appointments.js";
 import { ClientsPage } from "./pages/Clients.js";
 import { ServicesPage } from "./pages/Services.js";
 import { StaffPage } from "./pages/Staff.js";
+import { InvoicesPage } from "./pages/Invoices.js";
 
 const NAV_LINKS = [
   { to: "/", label: "Appointments" },
   { to: "/clients", label: "Clients" },
   { to: "/services", label: "Services" },
   { to: "/staff", label: "Staff" },
+  { to: "/invoices", label: "Invoices" },
 ];
 
 export function App() {
@@ -54,6 +56,7 @@ export function App() {
           <Route path="/clients" element={<ClientsPage />} />
           <Route path="/services" element={<ServicesPage />} />
           <Route path="/staff" element={<StaffPage />} />
+          <Route path="/invoices" element={<InvoicesPage />} />
         </Routes>
       </main>
     </div>