Add full light dark auto theme support to admin UI

[miolini]

What changed

This PR adds a real internal theme switcher for the Automata admin UI with three modes: auto, light, and dark.

The theme control is now part of the admin shell itself instead of feeling like a leftover framework widget. Administrators can change appearance directly from the internal UI, and the selected mode is reflected consistently across the whole admin experience.

UX improvements

The goal of this change is not only to expose a theme toggle, but to make the entire interface actually support it.

The admin UI now behaves like a complete themed product experience:

• Auto follows system preference
• Light forces the light interface
• Dark forces the dark interface
• the active mode is visually reflected in the switch itself
• cards, sidebar, forms, tables, alerts, buttons, and login screen all adapt coherently

Implementation details

The work is centered in the layout and shared admin styling layer.

Theme handling in the root layout now stores explicit mode selection and resolves auto against the system color scheme. The admin shell exposes the switch in the sidebar so it is available on all admin pages. The custom admin CSS was refactored from fixed light colors to shared theme variables so the design remains intentional across modes instead of only partially changing.

Why this matters

Before this change, theme support existed only in a partial technical sense. The admin UI had a hardcoded visual language that was mostly designed for light mode.

This PR makes theme support product quality instead of framework quality. It should feel native to the admin experience and usable for real operators who prefer light, dark, or automatic behavior.

Validation

• mix compile