Fix project site 404 rendering

[johnnygreco]

Summary

• replace the theme-dependent 404 with a self-contained responsive page
• inline the OpenShell mark and all light/dark styles so missing paths make no asset requests
• preserve distinct messages for ordinary missing pages and expired PR previews
• add a rendered-output regression test to the strict docs build

Root cause

Zensical renders its special 404.html without a normal page URL context. Extending the main theme therefore emitted domain-root paths such as /assets/..., while this project is hosted under /OpenShell-Research/. Those requests returned 404, leaving raw theme controls and oversized broken image placeholders.

Validation

• python3 tests/test_render_dev_notes.py — 11 passed
• python3 tests/test_docs_404.py — source checks passed
• scripts/build-docs.sh — clean strict build and rendered 404 regression checks passed
• YAML parsing and actionlint for both preview workflows
• Node execution checks for general and expired-preview path behavior
• local missing-preview route served the standalone generated page

[github-actions]

Documentation preview

The preview has been removed.