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/locales/en.json

@@ -436,6 +436,11 @@
   "common.none_no_cspt": "None (no processing template)",
   "common.none_no_input": "None (no input source)",
   "common.none_own_speed": "None (use own speed)",
+  "common.undo": "Undo",
+  "validation.required": "This field is required",
+  "bulk.processing": "Processing…",
+  "api.error.timeout": "Request timed out — please try again",
+  "api.error.network": "Network error — check your connection",
   "palette.search": "Search…",
   "section.filter.placeholder": "Filter...",
   "section.filter.reset": "Clear filter",