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/test-stream.html

@@ -1,9 +1,9 @@
 <!-- Test Source Modal -->
-<div id="test-stream-modal" class="modal">
+<div id="test-stream-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="test-stream-modal-title">
     <div class="modal-content">
         <div class="modal-header">
-            <h2 data-i18n="streams.test.title">Test Source</h2>
-            <button class="modal-close-btn" onclick="closeTestStreamModal()" title="Close">&#x2715;</button>
+            <h2 id="test-stream-modal-title" data-i18n="streams.test.title">Test Source</h2>
+            <button class="modal-close-btn" onclick="closeTestStreamModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
         </div>
         <div class="modal-body">
             <div class="form-group">