Add an Appearance section to the Personal tab in user settings with
three clearly labelled options (Auto/Light/Dark). A ThemeSelector hook
reads localStorage to highlight the active choice and updates it when
selection changes. Remove the dropdown toggle from both the main nav
and admin nav.
wire time_format into scope, add settings UI selector, change defaults
to 24h, and replace all user-facing Calendar.strftime calls with
centralized TimeHelpers using the user's timezone and time format.
