feat: add pet size/coat to booking flow with buffer-aware availability

- Add petSizeCategory and petCoatType dropdowns to booking wizard
  (after breed field, optional but encouraged)
- Pass selected values to GET /availability as query params
- large/x-large pets add service.defaultBufferMinutes to slot calculation
  and appointment end time (buffer never shown to client)
- POST /appointments saves size/coat to pet record
- Confirmation step shows total duration (service + buffer if applicable)

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

packages/types/src/index.ts

@@ -64,6 +64,7 @@ export interface Service {
   description: string | null;
   basePriceCents: number;
   durationMinutes: number;
+  defaultBufferMinutes: number;
   active: boolean;
   createdAt: string;
   updatedAt: string;