Frontend: structured error handling, state fixes, accessibility, i18n

- Enhance fetchWithAuth with auto-401, retry w/ exponential backoff, timeout
- Remove ~40 manual 401 checks across 10 feature files
- Fix state: brightness cache setter, manual edit flag resets, static import
- Add ARIA: role=dialog/tablist, aria-modal, aria-labelledby, aria-selected
- Add focus trapping in Modal base class, aria-expanded on hint toggles
- Fix WCAG AA color contrast with --primary-text-color variable
- Add i18n pluralization (CLDR rules for en/ru), getCurrentLocale export
- Replace hardcoded strings in dashboard.js and profiles.js
- Add data-i18n-aria-label support, 20 new keys in en.json and ru.json

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/templates/modals/profile-editor.html

@@ -1,9 +1,9 @@
 <!-- Profile Editor Modal -->
-<div id="profile-editor-modal" class="modal">
+<div id="profile-editor-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="profile-editor-title">
     <div class="modal-content">
         <div class="modal-header">
             <h2 id="profile-editor-title" data-i18n="profiles.add">📋 Add Profile</h2>
-            <button class="modal-close-btn" onclick="closeProfileEditorModal()" title="Close">&#x2715;</button>
+            <button class="modal-close-btn" onclick="closeProfileEditorModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
         </div>
         <div class="modal-body">
             <form id="profile-editor-form">
@@ -67,8 +67,8 @@
             </form>
         </div>
         <div class="modal-footer">
-            <button class="btn btn-icon btn-secondary" onclick="closeProfileEditorModal()" title="Cancel">&#x2715;</button>
-            <button class="btn btn-icon btn-primary" onclick="saveProfileEditor()" title="Save">&#x2713;</button>
+            <button class="btn btn-icon btn-secondary" onclick="closeProfileEditorModal()" title="Cancel" data-i18n-aria-label="aria.cancel">&#x2715;</button>
+            <button class="btn btn-icon btn-primary" onclick="saveProfileEditor()" title="Save" data-i18n-aria-label="aria.save">&#x2713;</button>
         </div>
     </div>
 </div>