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>

packages/db/src/schema.ts

@@ -133,6 +133,10 @@ export const appointments = pgTable("appointments", {
   staffId: uuid("staff_id").references(() => staff.id, {
     onDelete: "set null",
   }),
+  // Optional secondary staff (bather/assistant) for tip-split tracking
+  batherStaffId: uuid("bather_staff_id").references(() => staff.id, {
+    onDelete: "set null",
+  }),
   status: appointmentStatusEnum("status").notNull().default("scheduled"),
   startTime: timestamp("start_time").notNull(),
   endTime: timestamp("end_time").notNull(),
@@ -184,6 +188,20 @@ export const invoiceLineItems = pgTable("invoice_line_items", {
   createdAt: timestamp("created_at").notNull().defaultNow(),
 });
 
+// Per-staff tip allocation calculated when an invoice is paid.
+// staff_name is snapshotted at calculation time so reports remain accurate if staff is deleted.
+export const invoiceTipSplits = pgTable("invoice_tip_splits", {
+  id: uuid("id").primaryKey().defaultRandom(),
+  invoiceId: uuid("invoice_id")
+    .notNull()
+    .references(() => invoices.id, { onDelete: "cascade" }),
+  staffId: uuid("staff_id").references(() => staff.id, { onDelete: "set null" }),
+  staffName: text("staff_name").notNull(),
+  sharePct: numeric("share_pct", { precision: 5, scale: 2 }).notNull(),
+  shareCents: integer("share_cents").notNull(),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+});
+
 // Tracks which reminder emails have been sent per appointment (prevents duplicates).
 // reminder_type values: "confirmation", "24h", "2h"
 export const reminderLogs = pgTable(