feat: tip and payment splitting between staff roles (#34)

* feat: multi-groomer calendar view with per-groomer filtering

Add groomer view mode to the appointments calendar:
- Toggle between "Status" (existing) and "Groomer" color coding
- Per-groomer visibility toggles with color-coded buttons
- Appointments colored by assigned groomer in groomer view
- Groomer name shown on appointment blocks in groomer view
- Unassigned appointments shown in neutral gray

Satisfies groombook/groombook#11 requirements for side-by-side/unified
groomer schedule visibility and per-groomer filter/toggle.

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

* feat: tip and payment splitting between staff roles

Implements groombook/groombook#12 — track which staff worked on each
pet and calculate tip distribution based on who was involved.

Changes:
- DB: Add bather_staff_id to appointments (optional secondary staff)
- DB: Add invoice_tip_splits table (per-staff tip share ledger)
- API: appointments POST/PATCH accept batherStaffId
- API: GET /invoices/:id now includes tipSplits[]
- API: POST /invoices/:id/tip-splits — saves tip distribution
- API: GET /reports/tip-splits — payroll summary of tip earnings
- Frontend: Bather/Assistant select on New Appointment form
- Frontend: Tip Distribution section in Invoice Detail modal
  - Auto-populates 70%/30% split when bather is assigned
  - Editable percentages before payment; saved on Mark as Paid
  - Displays recorded splits on paid invoices

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

* fix: remove unused staff import from invoices route

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

---------

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

apps/api/src/routes/appointments.ts

@@ -26,6 +26,7 @@ const createAppointmentSchema = z.object({
   petId: z.string().uuid(),
   serviceId: z.string().uuid(),
   staffId: z.string().uuid().optional(),
+  batherStaffId: z.string().uuid().optional(),
   startTime: z.string().datetime(),
   endTime: z.string().datetime(),
   notes: z.string().max(2000).optional(),
@@ -41,6 +42,7 @@ const createAppointmentSchema = z.object({
 
 const updateAppointmentSchema = z.object({
   staffId: z.string().uuid().nullable().optional(),
+  batherStaffId: z.string().uuid().nullable().optional(),
   status: z
     .enum([
       "scheduled",