feat(chat): add ChatBubble widget and mock data for home screen

- Add ChatBubble reusable widget for chat messages
- Add HomeMockData for chat list mock data
- Add HomeScreen widget tests
- Add AG-UI chat design and implementation plan docs
- Add friendship design docs
- Ignore backend/logs directory

.gitignore

@@ -287,6 +287,7 @@ infra/cloud/volcano/env/*.env
 .buildlog/
 .history
 /logs/
+backend/logs/
 # Docker volumes (local data)
 docker/supabase/volumes/db/data/
 infra/docker/volumes/db/data/