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/reports.ts

@@ -9,6 +9,7 @@ import {
   appointments,
   clients,
   invoices,
+  invoiceTipSplits,
   services,
   staff,
 } from "@groombook/db";
@@ -303,6 +304,37 @@ reportsRouter.get("/clients", async (c) => {
   });
 });
 
+// ─── Tip splits payroll report ────────────────────────────────────────────────
+// GET /api/reports/tip-splits?from=&to=
+// Aggregates tip earnings per staff member for the period
+
+reportsRouter.get("/tip-splits", async (c) => {
+  const db = getDb();
+  const from = parseDate(c.req.query("from"), defaultFrom());
+  const to = parseDate(c.req.query("to"), defaultTo());
+
+  const rows = await db
+    .select({
+      staffId: invoiceTipSplits.staffId,
+      staffName: invoiceTipSplits.staffName,
+      totalTipCents: sql<number>`SUM(${invoiceTipSplits.shareCents})::int`,
+      invoiceCount: sql<number>`COUNT(DISTINCT ${invoiceTipSplits.invoiceId})::int`,
+    })
+    .from(invoiceTipSplits)
+    .innerJoin(invoices, eq(invoiceTipSplits.invoiceId, invoices.id))
+    .where(
+      and(
+        eq(invoices.status, "paid"),
+        gte(invoices.paidAt, from),
+        lt(invoices.paidAt, to)
+      )
+    )
+    .groupBy(invoiceTipSplits.staffId, invoiceTipSplits.staffName)
+    .orderBy(sql`SUM(${invoiceTipSplits.shareCents}) DESC`);
+
+  return c.json({ from: from.toISOString(), to: to.toISOString(), rows });
+});
+
 // ─── CSV export ───────────────────────────────────────────────────────────────
 // GET /api/reports/export.csv?type=revenue|appointments|services&from=&to=