Send platform operators to the platform console after sign-in

The operator account is the owner of a placeholder business, so sign-in
landed it on that business's empty dashboard with a "Create your first
branch" nudge. Operators now land on /platform (sign-in and the root page),
the dashboard header says "Platform operator", and the nudge is hidden on
the operator's own business.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/account.test.ts

@@ -132,3 +132,16 @@ describe("moving staff between branches", () => {
     expect((await move(t.owner, t.branchB)).status).toBe(404);
   });
 });
+
+describe("where sign-in lands", () => {
+  it("sends the platform operator to the platform console, owners to the dashboard, cashiers to the till", async () => {
+    const t = await makeTenant();
+    await Promise.all([setPassword(t.owner, "owner-pass"), setPassword(t.cashierA, "cashier-pass")]);
+    expect((await json(await signIn(t.owner, "owner-pass"))).body.redirectTo).toBe("/admin/dashboard");
+    expect((await json(await signIn(t.cashierA, "cashier-pass"))).body.redirectTo).toBe("/terminal");
+
+    await db(() => User.updateOne({ _id: t.owner.id }, { email: "operator@aquaflow.test", platformAdmin: true }));
+    const operator = { ...t.owner, email: "operator@aquaflow.test" };
+    expect((await json(await signIn(operator, "owner-pass"))).body.redirectTo).toBe("/platform");
+  });
+});