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/graph-editor.ts

@@ -914,7 +914,7 @@ function _graphHTML(): string {
     // Only set size from saved state; position is applied in _initMinimap via anchor logic
     const mmStyle = mmRect?.width ? `width:${mmRect.width}px;height:${mmRect.height}px;` : '';
     return `
-        <div class="graph-container">
+        <div class="graph-container" tabindex="0" role="application" aria-label="${t('graph.title')}">
             <div class="graph-toolbar">
                 <span class="graph-toolbar-drag" title="Drag to move">⠿</span>
                 <button class="btn-icon" onclick="graphFitAll()" title="${t('graph.fit_all')}">