Take the terminal's role from the back office, not from which tab was clicked

The role split was right; only its source was wrong. Signing in matched what
was typed against two constants compiled into the app — admin@nearle.in and
cashier@nearle.in — so which shell a person got was a property of the *build*.
A shop could not add a third person, revoke either of the two it had, or stop
anyone with the APK reading both passwords out of it.

TerminalLogin survives unchanged in shape, because the shape was the good part:
one flag the shell reads, a session that decides it, and a cashier sign-out that
takes the catalogue with it while a supervisor's leaves it behind. Every
consumer — visibleModulesProvider, resolvedModuleProvider, the sidebar, the page
header, the sign-out dialog — is untouched. What changed is that the enum is now
only constructible from a session the back office signed, so there is no path
left where the terminal grants itself a permission the server did not send.

It reads `can_manage_staff` rather than the role name or id. app_roles holds six
rows for four distinct roles, a great many accounts carry a roleid that is not
in the table at all, and the name comes back blank for most of them. Matching on
either would mean shipping a copy of the role table in the app and keeping the
two in step for ever. One boolean, decided server-side, cannot drift. It
defaults to false, which matters on the restore path: a session saved by a build
that predates the field comes back as a cashier, never silently as an admin.

This also restores the sign-in layer itself — pos_auth_api, pos_session,
session_store, the staff import and the bearer token — which an earlier commit
removed wholesale from a stale checkout. Its parent was the commit that added
them, so the deletion was a bad merge rather than a decision; the terminal has
been running on the two constants since.

The login screen loses its role tabs and its credential prefill. You do not
choose what you are on the way in.

The opener is now matched on the back office user id rather than on the first
account with a matching role, so the first bill of a shift is attributed to
whoever actually signed in.

Tests: the smoke suite pinned only the supervisor shell, and it was passing for
the wrong reason — the fake session omitted can_manage_staff, and the sidebar it
asserted on was there because the role was hardcoded. Both halves are pinned now
and the fake is parameterised. widget_test.dart was the stock Flutter counter
template, restored by the same bad merge, testing a MyApp that has never existed
in this repo.

292 tests pass; analyzer reports no errors and no warnings.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/widget_test.dart

@@ -1,30 +0,0 @@
-// This is a basic Flutter widget test.
-//
-// To perform an interaction with a widget in your test, use the WidgetTester
-// utility in the flutter_test package. For example, you can send tap and scroll
-// gestures. You can also use WidgetTester to find child widgets in the widget
-// tree, read text, and verify that the values of widget properties are correct.
-
-import 'package:flutter/material.dart';
-import 'package:flutter_test/flutter_test.dart';
-
-import 'package:nearle_pos/main.dart';
-
-void main() {
-  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
-    // Build our app and trigger a frame.
-    await tester.pumpWidget(const MyApp());
-
-    // Verify that our counter starts at 0.
-    expect(find.text('0'), findsOneWidget);
-    expect(find.text('1'), findsNothing);
-
-    // Tap the '+' icon and trigger a frame.
-    await tester.tap(find.byIcon(Icons.add));
-    await tester.pump();
-
-    // Verify that our counter has incremented.
-    expect(find.text('0'), findsNothing);
-    expect(find.text('1'), findsOneWidget);
-  });
-}