fix: header avatar caret opens nothing (no dropdown menu)

[thnndnly]

Summary

When logged in, the caret next to the user avatar (top-right header) toggles its
direction on click but never opens a menu.

Steps to Reproduce

1. Log in.
2. Click the arrow next to the avatar.

Expected Behavior

A menu opens (e.g. Profile, Logout). Note there is currently no logout path in
the UI at all.

Actual Behavior

Only the arrow flips down→up; nothing else happens.

Technical notes

src/components/Header/UserProfile.tsx: the onClick only flips isCaretDown.
There is no dropdown state, no menu, no useClickOutside.

Decision needed

Implement the dropdown (which items?) or remove the caret until a menu exists. A
working dropdown pattern already exists in TrustLevelDropdown.tsx +
useClickOutside.