Add multi-vendor capability with admin vendor management

- Add resolveVendorId() helper — admin can pass ?vendorId= to scope
  catalog operations to any vendor; other roles locked to JWT vendorId
- Thread ?vendorId= through products, categories, taxes, events routes
- Add DELETE /vendors/:id (admin only) with cascade-safe guard:
  blocks if vendor has users or transactions; otherwise cascade-deletes
  EventProduct → EventTax → Event → Product → Tax → Category → Vendor
- Rewrite VendorPage: admin gets full CRUD list, vendor gets own settings
- Add VendorFilter shared component (admin-only dropdown)
- Integrate VendorFilter into Catalog, Users, and Events pages so admin
  can switch vendor context for all create/read operations

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/pages/EventsPage.tsx

@@ -1,6 +1,8 @@
 import React, { useEffect, useState, useCallback } from "react";
 import { api } from "../api/client";
+import { useAuth } from "../context/AuthContext";
 import { PageHeader } from "../components/PageHeader";
+import { VendorFilter } from "../components/VendorFilter";
 import { Table } from "../components/Table";
 import { Modal } from "../components/Modal";
 import { FormField, Btn } from "../components/FormField";
@@ -50,6 +52,8 @@ interface EventSummary {
 // ─── Main Page ──────────────────────────────────────────────────────────────
 
 export default function EventsPage() {
+  const { user } = useAuth();
+  const [vendorId, setVendorId] = useState(user?.vendorId ?? "");
   const [events, setEvents] = useState<Event[]>([]);
   const [total, setTotal] = useState(0);
   const [loading, setLoading] = useState(true);
@@ -61,7 +65,8 @@ export default function EventsPage() {
   const load = useCallback(async () => {
     setLoading(true);
     try {
-      const res = await api.get<ApiList<Event>>("/events?limit=50");
+      const q = vendorId && user?.role === "admin" ? `?vendorId=${encodeURIComponent(vendorId)}&limit=50` : "?limit=50";
+      const res = await api.get<ApiList<Event>>(`/events${q}`);
       setEvents(res.data);
       setTotal(res.pagination.total);
     } catch (err) {
@@ -69,7 +74,7 @@ export default function EventsPage() {
     } finally {
       setLoading(false);
     }
-  }, []);
+  }, [vendorId, user?.role]);
 
   useEffect(() => { load(); }, [load]);
 
@@ -120,11 +125,14 @@ export default function EventsPage() {
 
   return (
     <div style={{ padding: "32px 28px" }}>
-      <PageHeader
-        title="Events"
-        subtitle={`${total} event${total !== 1 ? "s" : ""}`}
-        action={<Btn onClick={() => { setEditing(null); setShowForm(true); }}>+ New Event</Btn>}
-      />
+      <div style={{ display: "flex", alignItems: "flex-start", justifyContent: "space-between", flexWrap: "wrap", gap: 12, marginBottom: 4 }}>
+        <PageHeader
+          title="Events"
+          subtitle={`${total} event${total !== 1 ? "s" : ""}`}
+          action={<Btn onClick={() => { setEditing(null); setShowForm(true); }}>+ New Event</Btn>}
+        />
+        <VendorFilter vendorId={vendorId} onChange={setVendorId} />
+      </div>
 
       <Table columns={columns} data={events} keyField="id" loading={loading} emptyText="No events yet." />