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/js/features/tabs.js

@@ -5,7 +5,11 @@
 import { apiKey, refreshInterval, setRefreshInterval, dashboardPollInterval } from '../core/state.js';
 
 export function switchTab(name) {
-    document.querySelectorAll('.tab-btn').forEach(btn => btn.classList.toggle('active', btn.dataset.tab === name));
+    document.querySelectorAll('.tab-btn').forEach(btn => {
+        const isActive = btn.dataset.tab === name;
+        btn.classList.toggle('active', isActive);
+        btn.setAttribute('aria-selected', String(isActive));
+    });
     document.querySelectorAll('.tab-panel').forEach(panel => panel.classList.toggle('active', panel.id === `tab-${name}`));
     localStorage.setItem('activeTab', name);
     if (name === 'dashboard') {