Address findings from the templates code review:
- Fix XSS in admin.html onclick handlers via data attributes
- Fix XSS in 5 edit page signal initializations via JSON serialization
- Fix register.html token exposure by moving to data attribute
- Add entity_icon, quick_notes_toggles, add_form_submit macros
- Replace hardcoded colors with design tokens (warning, error, success)
- Add warning design tokens to CSS theme
- Scope MutationObserver to main element
- Add defer to webauthn.js script tags
- Refactor login/register JS to arrow functions
- Guard lightbox script behind image_url check
- Fix else-if to elif in 5 templates
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