fix: replace incorrect logo.png with m8logo.svg

Removed logo.png (wrong logo). Added m8logo.svg from BrandingKit as
the correct M8Chat logo. Login screen now renders SVG via flutter_svg.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/features/auth/presentation/login_screen.dart

@@ -1,9 +1,10 @@
-// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-02
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-03
 // Login screen. Username + password only. No registration link.
 // Respects system theme preference.
 
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_svg/flutter_svg.dart';
 
 import '../../../core/config/app_config.dart';
 import 'login_controller.dart';
@@ -102,15 +103,10 @@ class _LogoSection extends StatelessWidget {
   Widget build(BuildContext context) {
     return Column(
       children: [
-        Image.asset(
-          'assets/images/logo.png',
+        SvgPicture.asset(
+          'assets/images/m8logo.svg',
           width: 96,
           height: 96,
-          errorBuilder: (_, __, ___) => CircleAvatar(
-            radius: 48,
-            backgroundColor: theme.colorScheme.primary,
-            child: const Icon(Icons.chat_bubble, size: 48, color: Colors.white),
-          ),
         ),
         const SizedBox(height: 20),
         Text(