feat: add color-mode toggle dropdown to default topbar (#6010)

The color-mode toggle was documented at docs/color-mode.html but not
visible by default in the topbar, so multiple users missed it and
opened bugs ('light/dark toggle is missing').

Adds a visible Light / Dark / Auto dropdown to the default topbar next
to the fullscreen toggle, with the wiring JavaScript shipped globally
in _scripts.astro so it works out of the box on every demo page.

Behaviour:

- Three options: Light, Dark, Auto
- Auto follows prefers-color-scheme and live-updates if the OS theme
  changes mid-session
- Persists the user's choice in localStorage under the 'lte-theme' key
- Active option is highlighted in the dropdown with a check mark
- The trigger icon swaps to reflect the current theme: sun (light) /
  moon (dark) / circle-half (auto)