Add user-facing 404 page and client-side deep-link guard #785

Add a static 404 page and a client-side guard in index.html that redirects deep-linked nested file paths (e.g. /example-games/hello-world/index.html) to /404.html. This prevents the SPA dev fallback from silently booting the app at the wrong base path and improves UX for invalid URLs.

Scope:

• Add public/404.html user-facing page.
• Add a small redirect guard in index.html before booting the app.
• Update docs/DEVELOPER.md to mention the 404 page and recommended dev URL.

Acceptance Criteria:

1. Requests for nested/nonexistent example HTML files are redirected to /404.html client-side in dev and preview modes.
2. public/404.html provides a helpful link back to the Game Selector root.
3. No regression in existing app boot flow or tests.

Related-to:CG-0MM5ZGB8U02S0BFO