feat(customers): full CRUD module — pattern for all other modules

Establishes the multi-tenant module pattern. Subsequent modules (services,
software, calendar, tasks, finance, invoices) will copy this structure.

Validation:
- lib/validation/customers.ts: Zod schema with Turkish messages, optional
  fields normalized to undefined.

Server actions (lib/appwrite/customer-actions.ts):
- createCustomerAction, updateCustomerAction, deleteCustomerAction
- All call requireTenant() guard, write team-scoped row permissions
  (read+update by team, delete by owner|admin), and emit audit log.
- Update/delete cross-check tenantId on the existing row before mutating
  (defense in depth even though row-level perms already enforce it).
- Field-level errors flattened from Zod for inline form display.

Server-side queries (lib/appwrite/customer-queries.ts):
- listCustomers(tenantId), getCustomer(tenantId, id) — admin SDK with
  Query.equal('tenantId',...) tenant scope.

UI:
- /customers page (server component): pulls active tenant context, lists
  customers, hands off to CustomersClient.
- CustomersClient: TanStack Table with global filter (name/email/phone/
  taxId), column sorting on name + createdAt, pagination (20/page),
  status badges, row actions (Edit/Delete dropdown), empty-state CTA.
- CustomerFormSheet: shadcn Sheet-based add/edit form with all fields,
  toast feedback (sonner), inline field errors. Reused for create + update
  by switching the action.
- DeleteCustomerDialog: confirmation modal with destructive button.

Infrastructure:
- Added sonner Toaster to root layout (richColors, closeButton).
- Updated metadata to 'İşletmem KovakCRM' and html lang='tr'.
- Renamed theme storage key to isletmem-ui-theme.

src/lib/validation/customers.ts

@@ -0,0 +1,16 @@
+import { z } from "zod";
+
+export const customerSchema = z.object({
+  name: z.string().trim().min(1, "İsim zorunlu.").max(255),
+  email: z
+    .union([z.string().email("Geçerli bir email girin."), z.literal("")])
+    .optional()
+    .transform((v) => (v ? v : undefined)),
+  phone: z.string().trim().max(30).optional().transform((v) => (v ? v : undefined)),
+  taxId: z.string().trim().max(50).optional().transform((v) => (v ? v : undefined)),
+  address: z.string().trim().max(500).optional().transform((v) => (v ? v : undefined)),
+  notes: z.string().trim().max(2000).optional().transform((v) => (v ? v : undefined)),
+  status: z.enum(["active", "passive"]).optional().default("active"),
+});
+
+export type CustomerInput = z.infer<typeof customerSchema>;