Users can now set their own profile picture instead of asking staff:
POST /api/auth/avatar/ accepts a J-ID (or blank to clear) and reuses the
same item resolution as the staff endpoint. The Account page gains a
profile picture card with a searchable, paginated library grid — any
item works (the thumbnail is used), the current avatar is marked, and
the choice is confirmed before saving. Refreshing the signed-in user
updates the shell avatar immediately.
Verified against the dev server: set, clear, unknown J-ID -> 400,
anonymous -> 401.