feat(identity): one image — the site photo is the only avatar input
Removed the separate account-avatar upload. A user's avatar everywhere (nav, account, profile
header, fediverse) is now their SITE photo, set in one place (site settings). The account page
shows it read-only with a link.
- middleware/render.js — nav avatar = site photo always; drop the account-avatar cross-fallback
- views/pages/account.ejs — remove the avatar upload; read-only site photo + hint to site settings
- services/i18n.js — acct.photo_site_hint (nl/en/de)