Remove unused chat directory and related files

- Removed entire frontend/app/view/chat/ directory (13 files)
- Removed avatar.tsx, avatar.scss, avatar.stories.tsx
- Removed collapsiblemenu.tsx, collapsiblemenu.scss, collapsiblemenu.stories.tsx
- Removed avatar-dims-mixin from mixins.scss

All files were verified to be unused outside their own directory.

Co-authored-by: sawka <2722291+sawka@users.noreply.github.com>

Author: Copilot