refactor(screenshots): commit avatar fixtures and remove local path dependency

Move the 10 seed user avatars from /home/anna/Downloads/tp/avatar/ into
playwright/fixtures/avatars/<uid>/avatar.png so the suite has no remaining
dependency on local machine paths outside the repo.

Update seed/users.ts to reference AVATARS fixture dir instead of AVATAR_DIR.

AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala