feat(GRO-1792): add recovery paths to booking error and cancellation pages

- Add "Start a new booking" button to BookingError linking to /admin/book
- Add "Book again" button to BookingCancelled linking to /admin/book
- Add business contact info section to BookingError (from BUSINESS_CONTACT_INFO constant)
- Replace hardcoded colors with CSS variables (--color-error, --color-cancelled, etc.)
- Add page-level string constants to eliminate hardcoded strings
- Add unit tests for both pages (9 tests passing)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/lib/contact.ts

@@ -0,0 +1,7 @@
+// Business contact information — update values to reflect actual business details.
+// Used on error/cancellation pages to help customers reach the business.
+export const BUSINESS_CONTACT_INFO = {
+  phone: "(555) 000-1234",
+  email: "hello@groombook.example.com",
+  address: "123 Main St, Anytown, USA",
+} as const;