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/login_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/client.dart';
 import '../api/endpoints/auth.dart';
@@ -31,7 +32,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
       final url = await ref.read(serverUrlProvider.future);
       if (url == null) {
         if (!mounted) return;
-        Navigator.of(context).pushReplacementNamed('/server-url');
+        context.go('/server-url');
         return;
       }
       final dio = ApiClient.buildDio(
@@ -47,7 +48,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
             userJson: res.rawUserJson,
           );
       if (!mounted) return;
-      Navigator.of(context).pushReplacementNamed('/home');
+      context.go('/home');
     } on DioException catch (e) {
       final code = ApiError.fromDio(e).code;
       final copy = (await ErrorCopy.load()).forCode(code);
@@ -83,7 +84,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                   : const Text('Sign in'),
             ),
             TextButton(
-              onPressed: () => Navigator.of(context).pushReplacementNamed('/server-url'),
+              onPressed: () => context.go('/server-url'),
               child: const Text('Change server URL'),
             ),
           ]),

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 (_) {