Frontend improvements: CSS foundations, accessibility, UX enhancements
CSS: Add design token variables (spacing, timing, weights, z-index layers),
migrate all hardcoded z-index to named vars, fix light theme contrast for
WCAG AA, add skeleton loading cards, mask-composite fallback, card padding.
Accessibility: aria-live on toast, aria-label on health dots, sr-only class,
graph container keyboard focusable, MQTT password wrapped in form element.
UX: Modal auto-focus on open, inline field validation with blur, undo toast
with countdown, bulk action progress indicator, API error toast on failure.
i18n: Add common.undo, validation.required, bulk.processing, api.error.*
keys in EN/RU/ZH.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
alexei.dolgolyov
47c696bae3