Add tabbed settings page with Profile, Appearance, Notifications, and
Providers sections. Profile name/avatar save via BetterAuth updateUser.
Appearance and notification preferences save to memory preferences API
(ui.theme, ui.sidebar_collapsed, ui.default_model, notify.*). Settings
load from API on tab activation with success/error feedback on save.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>