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/app.dart

@@ -1,18 +1,19 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import 'shared/routing.dart';
 import 'theme/theme_data.dart';
 
-class MinstrelApp extends StatelessWidget {
+class MinstrelApp extends ConsumerWidget {
   const MinstrelApp({super.key});
 
   @override
-  Widget build(BuildContext context) {
-    return MaterialApp(
+  Widget build(BuildContext context, WidgetRef ref) {
+    final router = buildRouter(ref);
+    return MaterialApp.router(
       title: 'Minstrel',
       theme: buildThemeData(),
-      home: const Scaffold(
-        body: Center(child: Text('Minstrel — scaffold')),
-      ),
+      routerConfig: router,
     );
   }
 }