Add web UI dark mode toggle

[mriley-cardinal]

Summary

• add light/dark theme CSS variables to the standalone Strix web UI
• add a header theme toggle that persists to localStorage
• default to prefers-color-scheme: dark when no saved preference exists
• cover the dark-mode UI pieces in existing web UI tests

Testing

• python -m py_compile open_strix/web_ui.py
• targeted pytest checks for web UI rendering and static assets passed locally

Note: full tests/test_web_ui.py has existing Windows-only failures in shell-job tests because bash is unavailable in this environment; those are unrelated to this patch.

[mriley-cardinal]

Thanks — I'll keep this PR focused on the original web UI dark-mode toggle scope and leave adaptive/theme-aware agent HTML handling for a separate follow-up discussion/PR.