Add dark mode with Light, Dark and System choices

[ultimatecoder]

Closes #117.

What

• Header button cycles Light → Dark → System. The icon and aria-label show the current choice.
• The choice is saved in localStorage. System is the default and follows the OS setting, switching live when it changes.
• Mobile browser toolbar color (theme-color) is #424242 in light, as before, and #1b1b1d in dark.

How

• Minima 2.5 compiles its colors into the CSS and runs lighten/darken on them, so its variables can't become CSS custom properties. Dark mode is a set of override rules under html[data-theme="dark"] in _sass/dark-theme.scss.
• An inline script in _includes/head.html sets data-theme before the stylesheet paints, so there is no light flash. assets/js/theme.js handles the button, the OS change event and syncing across tabs.
• Without JavaScript the button stays hidden and a prefers-color-scheme media query follows the OS.
• Code blocks use rouge's github.dark colors (_sass/_syntax-dark.scss). The dark palette meets WCAG AA contrast (text 13.4:1, links 7.1:1, meta 6.6:1).
• Colors fade only right after a click (html.theme-transition), never on page load.

Verification

• ./scripts/test passes (build + htmlproofer).
• Compiled main.css compared with master: 0 lines removed or changed, only additions, so light mode is unchanged.
• In the browser preview:
  • Clicking the button cycles the stored value (light, dark, system), the icon, the label and theme-color.
  • Dark persists across a page change on a light OS.
  • On System, switching the OS between light and dark updates the page without a reload.
  • No console errors, and no fade on load.
  • At mobile width the button sits in the hamburger menu, with no horizontal scroll.

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