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>

server/src/wled_controller/static/js/features/dashboard.ts

@@ -587,7 +587,8 @@ function renderDashboardTarget(target: any, isRunning: boolean, devicesMap: Reco
         let healthDot = '';
         if (isLed && state.device_last_checked != null) {
             const cls = state.device_online ? 'health-online' : 'health-offline';
-            healthDot = `<span class="health-dot ${cls}"></span>`;
+            const statusLabel = state.device_online ? t('device.health.online') : t('device.health.offline');
+            healthDot = `<span class="health-dot ${cls}" role="status" aria-label="${statusLabel}"></span>`;
         }
 
         const cStyle = cardColorStyle(target.id);