feat: add EN/VI language toggle — i18n for all 5 public pages

ui: default theme to light mode; unify footer across all pages

fix: use window.location.href as OAuth redirectTo so localhost and prod both work

feat: move theme toggle into account dropdown, add scroll-to-top float button

fix: use onAuthStateChange to detect OAuth redirect session in login.html

fix: restore /app/ mount, add / redirect to home.html

fix: serve frontend at / instead of /app/ for cleaner URLs

feat: always redirect to home.html after login regardless of role

feat: redirect to home.html after Google login (admin -> admin.html)

feat(frontend): account dropdown with Google name in header nav

fix(frontend): wire session-aware nav onto renamed pages after ntp merge

feat(auth+db): session-aware nav, ownership fix, admin hardening, schema cleanup