Add global dynamic theme hue slider to Settings

[Copilot]

This change introduces a dynamic theme control so players can shift the game’s color styling at runtime from Settings. The selected theme hue is applied globally and restored across sessions.

• Theme control in Settings

  • Added a new Theme hue range slider to .settings-menu-ui > .container.
  • Slider is injected once when Settings UI appears, without modifying bundled game code paths.

• Global runtime theming

  • Added a dedicated mod script (mods/dynamic_theme.js) and loaded it from index.html.
  • The script sets a root CSS variable (--cwctrack-theme-hue) and applies:
    • filter: hue-rotate(var(--cwctrack-theme-hue))
    • to #screen, #ui, and #transition-layer so the theme affects the full game presentation.

• Persistence

  • Hue value is stored in localStorage (cwctrackThemeHue) and restored on startup.
  • Input values are clamped to 0..360 before apply/store.

• Focused automated coverage

  • Extended tests/main-menu.spec.js with a targeted test that verifies the slider exists and updates the theme CSS variable.

slider.addEventListener("input", () => {
  const hue = clampHue(Number(slider.value));
  document.documentElement.style.setProperty("--cwctrack-theme-hue", `${hue}deg`);
  localStorage.setItem("cwctrackThemeHue", String(hue));
});