Feature: Add persisted light/dark theme switcher for docs

[hdcodedev]

Summary

Implement a real theme switcher and activate full light/dark support.

Why

Light theme variables exist but the app is hard-locked to dark mode at the root HTML level.

Scope

• Add a theme toggle control in docs shell (header or settings area).
• Persist user preference (localStorage/cookie).
• Respect system preference on first load.
• Ensure all major components render correctly in both themes.

Acceptance Criteria

• Theme can be switched without code changes/redeploy.
• Preference persists across reloads.
• No unreadable elements in either theme.
• Theme behavior is documented in README or docs-app notes.

[hdcodedev]

Closed — addressed or superseded by modernization work in #44.