Persist the app UI language so emails and notifications can follow it.

profile_data['ui_language'] is read by five i18n consumers (notification, email, booking, school enrolment, scam-removal) but until 2026-08-25 was written by NOTHING, so every email fell back to English. The signup path now writes it once; this keeps it current when the user switches language in the app (LanguageContext calls it fire-and-forget).

A dedicated endpoint rather than a key on the big profile PUT on purpose: that handler ignores unknown fields silently, which is exactly how the /privacy-settings email toggles became a no-op.