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/templates/index.html

@@ -37,6 +37,7 @@
                 <select id="locale-select" onchange="changeLocale()" data-i18n-title="locale.change" title="Change language" style="padding: 4px 8px; border: 1px solid var(--border-color); border-radius: 4px; background: var(--bg-color); color: var(--text-color); font-size: 0.8rem; cursor: pointer;">
                     <option value="en">English</option>
                     <option value="ru">Русский</option>
+                    <option value="zh">中文</option>
                 </select>
                 <button id="login-btn" class="btn btn-primary" onclick="showLogin()" style="display: none; padding: 4px 12px; font-size: 0.8rem;">
                     🔑 <span data-i18n="auth.login">Login</span>