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/static/css/base.css

@@ -18,6 +18,7 @@
     --text-color: #e0e0e0;
     --border-color: #404040;
     --display-badge-bg: rgba(0, 0, 0, 0.4);
+    --primary-text-color: #66bb6a;
     color-scheme: dark;
 }
 
@@ -28,6 +29,7 @@
     --text-color: #333333;
     --border-color: #e0e0e0;
     --display-badge-bg: rgba(255, 255, 255, 0.85);
+    --primary-text-color: #3d8b40;
     color-scheme: light;
 }