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/templates/modals/settings.html

@@ -181,6 +181,7 @@
 
             <!-- ═══ MQTT tab ═══ -->
             <div id="settings-panel-mqtt" class="settings-panel">
+                <form onsubmit="saveMqttSettings(); return false;" autocomplete="off">
                 <div class="form-group">
                     <div class="label-row">
                         <label data-i18n="settings.mqtt.label">MQTT</label>
@@ -227,8 +228,9 @@
                         </div>
                     </div>
 
-                    <button class="btn btn-primary" onclick="saveMqttSettings()" style="width:100%" data-i18n="settings.mqtt.save">Save MQTT Settings</button>
+                    <button type="submit" class="btn btn-primary" style="width:100%" data-i18n="settings.mqtt.save">Save MQTT Settings</button>
                 </div>
+                </form>
             </div>
 
             <div id="settings-error" class="error-message" style="display:none;"></div>