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>

packages/types/src/index.ts

@@ -91,6 +91,7 @@ export interface Appointment {
   petId: string;
   serviceId: string;
   staffId: string | null;
+  batherStaffId: string | null;
   status: AppointmentStatus;
   startTime: string;
   endTime: string;
@@ -103,6 +104,16 @@ export interface Appointment {
   updatedAt: string;
 }
 
+export interface InvoiceTipSplit {
+  id: string;
+  invoiceId: string;
+  staffId: string | null;
+  staffName: string;
+  sharePct: string;
+  shareCents: number;
+  createdAt: string;
+}
+
 export type InvoiceStatus = "draft" | "pending" | "paid" | "void";
 export type PaymentMethod = "cash" | "card" | "check" | "other";
 
@@ -131,6 +142,7 @@ export interface Invoice {
   createdAt: string;
   updatedAt: string;
   lineItems?: InvoiceLineItem[];
+  tipSplits?: InvoiceTipSplit[];
 }
 
 // Paginated list response