ndg: add a theme toggle to the page header

[KiaraGrouwstra]

This adds a button to the page header that selects the light or the dark theme.

• A click selects the theme that is not shown. The page keeps the selection in localStorage and applies it before the first paint, so the page does not flash in the other theme.
• When the selected theme is the same as the system theme, the page removes the selection and follows the system theme again. Thus the toggle has only two states for the reader, but a reader who never clicks it always gets the system theme.
• The style sheet applies each prefers-color-scheme rule also when data-theme on <html> selects that theme, and does not apply it when data-theme selects the other theme.
• The toggle also selects the matching <source> of a <picture> with a prefers-color-scheme media query, so light and dark screenshots follow the page theme.
• The page sends an ndg:themechange event on document with { mode, resolved } when the theme changes, so other scripts can follow the theme.
• The icons are a sun and a moon, made of simple SVG shapes in the template. They do not come from an icon set.

This change is independent of #300 (Mermaid diagrams). When both are merged, Mermaid can draw the diagrams again on ndg:themechange. I will add that change to the pull request that is merged second.

Tests: deno test -A passes, deno lint is clean and nix build .#ndg passes. I also checked the pages in a headless Chromium with a light and a dark system theme, before and after a click. deno fmt --check reports one file, which is the same on main.

Assisted-by: Claude:claude-opus-5-5

https://claude.ai/code/session_01RJvbghuj5GtCBiCjmBepNJ

[NotAShelf]

Simple enough that I'll go ahead and merge.

#300 needs a little bit of work, but I will have a review for it shortly.