Replace the single-icon popover with a dropdown of radio inputs with a
compact two-button toggle (light/dark), inspired by vale.rocks' design:
clicking an icon forces that theme, clicking the active icon again
returns to following the system preference. Keeps the existing
light-dark() colour-scheme mechanism and view-transition wipe
animation, and reuses the project's existing Fluent UI sun/moon icons.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fn4aNjE3ofpDM78F4qmdCV
