feat(flutter): GoRouter shell + version gate + auth-aware redirects

Cold launch flow: no server-url -> /server-url. URL set, no token ->
/login. Token present -> /home with shell. VersionGate runs once when
the URL changes, hits /healthz, blocks if min_client_version > package
version. /home is a placeholder until Task 14.

Replaces Navigator.pushReplacementNamed in server_url_screen +
login_screen with context.go now that go_router owns the routes.

flutter_client/lib/auth/server_url_screen.dart

@@ -1,6 +1,7 @@
 import 'package:dio/dio.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
 
 import '../api/endpoints/health.dart';
 import '../theme/theme_extension.dart';
@@ -36,7 +37,7 @@ class _ServerUrlScreenState extends ConsumerState<ServerUrlScreen> {
       }
       await ref.read(authControllerProvider.notifier).setServerUrl(url);
       if (!mounted) return;
-      Navigator.of(context).pushReplacementNamed('/login');
+      context.go('/login');
     } on DioException catch (_) {
       setState(() => _error = "Couldn't reach that server.");
     } catch (_) {