feat: Matrix login flow with auth guard and client provider

lib/main.dart

@@ -1,8 +1,32 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:pyramid/core/app.dart';
+import 'package:pyramid/core/matrix_client.dart';
 
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
-  runApp(const ProviderScope(child: PyramidApp()));
+  runApp(const ProviderScope(child: PyramidBootstrap()));
+}
+
+class PyramidBootstrap extends ConsumerWidget {
+  const PyramidBootstrap({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final clientAsync = ref.watch(matrixClientProvider);
+
+    return clientAsync.when(
+      loading: () => const MaterialApp(
+        home: Scaffold(
+          body: Center(child: CircularProgressIndicator()),
+        ),
+      ),
+      error: (e, _) => MaterialApp(
+        home: Scaffold(
+          body: Center(child: Text('Fehler: $e')),
+        ),
+      ),
+      data: (_) => const PyramidApp(),
+    );
+  }
 }