feat: new 404 page, theme selector, and post-launch QA fixes - #3108

0 findings

This PR has no effect on the security posture. It is a UI and theming refactor: Heroicons are replaced with in-repo SVG icons, Dark Mode becomes a System/Light/Dark submenu, the 404 page is restyled, and colour tokens are updated. Language and theme selection remain allowlisted (hardcoded en/es/fr/el and system/light/dark); the 404 home link is a fixed /; and no new user-controlled sinks, redirects, or injection paths were introduced.