Remove unused chat directory and related components (#2479)

Removes the entire `frontend/app/view/chat/` directory and its
dependencies—avatar and collapsiblemenu components—which were only
referenced within the chat code itself.

**Removed:**
- `frontend/app/view/chat/` directory (13 files: channels, chat,
chatbox, chatmessages, userlist + their styles/stories)
- `frontend/app/element/avatar.{tsx,scss,stories.tsx}` (only used by
chat/userlist)
- `frontend/app/element/collapsiblemenu.{tsx,scss,stories.tsx}` (only
used by chat/channels)
- `avatar-dims-mixin` from `frontend/app/mixins.scss` (only used by
avatar.scss)

Total: 19 files, ~1300 lines removed.


---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sawka <2722291+sawka@users.noreply.github.com>

Author: Copilot