Add Chinese locale, fix audio device duplicates, remove display lock restriction

- Add Simplified Chinese (中文) locale with all 906 translation keys
- Fix duplicate audio devices: WASAPI two-pass enumerate avoids double-listing
  loopback endpoints; cross-engine dedup by (name, is_loopback) prefers
  higher-priority engine
- Remove redundant display lock check from capture template, picture source,
  and postprocessing template test endpoints — screen capture is read-only
  and concurrent access is safe

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

server/src/wled_controller/static/js/core/i18n.js

@@ -8,7 +8,8 @@ let _initialized = false;
 
 const supportedLocales = {
     'en': 'English',
-    'ru': 'Русский'
+    'ru': 'Русский',
+    'zh': '中文'
 };
 
 const fallbackTranslations = {