An IconButton between the notifications bell and the user menu cycles
the theme mode in radio order (sun/moon/monitor mirror the CURRENT
choice). New useThemeMode() hook is the single write path (persist +
apply + same-document event), so the settings radios and the toggle
stay in sync; AppearanceSection now uses it too. Also rendered for
signed-out visitors — the mode is a device-local preference. i18n de+en;
unit tests for cycle/setter, theme.spec covers cycling, radio sync,
persistence, and the signed-out top bar.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QRtCnB3uLdQtFmvp9HXcRX
