Remove unused chat directory and related components

[Copilot]

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.

Original prompt

Can you verify that everything in frontend/app/view/chat is not used? if so remove all the files. i also think avatar.tsx is then unused... if so, remove that too... and then i also thnk collapsibleMenu is unused? finally if we can remove avatar, i think we can remove avatar-dims-mixin from mixins.scss...

(do not modify any go.mod go.sum files -- e.g. in the tsunami directory as those files are not part of the frontend or this task)

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[coderabbitai]

Important

Review skipped

Bot user detected.

To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}