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>

pubspec.yaml

@@ -46,6 +46,7 @@ dependencies:
   cached_network_image: ^3.4.0
   timeago: ^3.7.0
   intl: ^0.20.0
+  flutter_svg: ^2.2.4
 
 dev_dependencies:
   flutter_test: