feat(claude): account login pane — host claude login in a modal terminal (T-485)

Completes the `login` verb + the UI add/re-login affordances. The
accountActionChannel 'login' action opens ClaudeLoginDialog — a modal hosting a
TerminalPane that runs `CLAUDE_CONFIG_DIR=<dir> claude login`, so the CLI drives
the OAuth browser flow and credentials land in that account's config dir (D-64:
one CLI-initiated browser flow, on explicit action).

TerminalPane gains optional argv/env/cwdOverride (default stays the login
shell), and its pane.spawn carries env when set. The login pane reuses the
TerminalPane *widget* — a code import, not a runtime dependency, since it spawns
through the always-present pane.spawn IPC rather than the terminal extension.

Tests cover the parameterized spawn args and the dialog's host wiring (title,
CLAUDE_CONFIG_DIR, close). Closes T-485.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/claude/account_login_dialog_test.dart

@@ -0,0 +1,43 @@
+/// T-485: the account login dialog hosts `claude login` in a terminal pane with
+/// the account's CLAUDE_CONFIG_DIR, and its close affordance dismisses. The CLI
+/// owns the OAuth flow; this verifies the host wiring (title, env, close).
+library;
+
+import 'package:clide/builtin/claude/src/account_login_dialog.dart';
+import 'package:clide/builtin/terminal/src/terminal_pane.dart';
+import 'package:clide/clide.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../../helpers/kernel_fixture.dart';
+import '../../helpers/widget_harness.dart';
+
+void main() {
+  late KernelFixture fixture;
+  setUp(() async => fixture = await KernelFixture.create());
+  tearDown(() async => fixture.dispose());
+
+  testWidgets('renders the account title, spawns claude login with the config dir, and closes', (tester) async {
+    var closed = false;
+    Map<String, Object?>? spawnArgs;
+    fixture.ipc.setConnected(true);
+    fixture.ipc.stub('pane.spawn', (args) async {
+      spawnArgs = args;
+      return IpcResponse.ok(id: 'r1', data: {'id': 'p1', 'pid': 1});
+    });
+    fixture.ipc.stub('pane.close', (args) async => IpcResponse.ok(id: 'r2'));
+
+    await tester.pumpWidget(harness(fixture, ClaudeLoginDialog(name: 'work', dir: '/home/u/.claude-work', onClose: () => closed = true)));
+    await pumpAsync(tester);
+
+    expect(find.text('Sign in: work'), findsOneWidget);
+    final pane = tester.widget<TerminalPane>(find.byType(TerminalPane));
+    expect(pane.argv, ['claude', 'login']);
+    expect(pane.env, {'CLAUDE_CONFIG_DIR': '/home/u/.claude-work'});
+    expect(spawnArgs?['env'], {'CLAUDE_CONFIG_DIR': '/home/u/.claude-work'});
+
+    await tester.tap(find.byKey(const Key('account-login-close')));
+    await tester.pump();
+    expect(closed, isTrue);
+  });
+}

test/builtin/terminal/terminal_pane_test.dart

@@ -75,6 +75,25 @@ void main() {
     expect(spawnedCwd, isNot(Directory.current.path));
   });
 
+  testWidgets('a caller-supplied argv + env + cwd reach pane.spawn (T-485)', (tester) async {
+    Map<String, Object?>? spawnArgs;
+    fixture.ipc.setConnected(true);
+    fixture.ipc.stub('pane.spawn', (args) async {
+      spawnArgs = args;
+      return IpcResponse.ok(id: 'r1', data: {'id': 'pane-1', 'pid': 1});
+    });
+    fixture.ipc.stub('pane.close', (args) async => IpcResponse.ok(id: 'r2'));
+
+    await tester.pumpWidget(
+      harness(fixture, const TerminalPane(argv: ['claude', 'login'], env: {'CLAUDE_CONFIG_DIR': '/home/u/.claude-work'}, cwdOverride: '/repo')),
+    );
+    await pumpAsync(tester);
+
+    expect(spawnArgs?['argv'], ['claude', 'login']);
+    expect(spawnArgs?['env'], {'CLAUDE_CONFIG_DIR': '/home/u/.claude-work'});
+    expect(spawnArgs?['cwd'], '/repo');
+  });
+
   testWidgets('disposing before spawn completes sends no close', (tester) async {
     final closed = <String>[];
     fixture.ipc.setConnected(false); // spawn bails out: no pane id