i18n: add Arabic (ar-SA) UI locale with RTL support

Add Modern Standard Arabic (ar-SA) as a selectable UI language alongside
the existing zh-CN, en-US and ru-RU locales.

- web/static/i18n/ar-SA.json: new Arabic locale (4272 leaf strings,
  full key/placeholder parity with en-US). The `c2` and `webshell`
  sections are intentionally omitted and fall back to English via the
  i18next fallback chain (ar-SA -> en-US -> zh-CN).
- web/static/js/i18n.js: register ar-SA, detect `ar*` navigator language,
  preload en-US as fallback, set document dir="rtl" for Arabic, and add
  the ar-SA language label handling.
- web/static/i18n/{zh-CN,en-US,ru-RU}.json: add the `lang.arSA` label.
- web/templates/{index,api-docs}.html: add the العربية option to the
  language dropdown.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/static/i18n/en-US.json

@@ -2,7 +2,8 @@
   "lang": {
     "zhCN": "中文",
     "enUS": "English",
-    "ruRU": "Русский"
+    "ruRU": "Русский",
+    "arSA": "العربية"
   },
   "mobile": {
     "openNav": "Open navigation menu",

web/static/i18n/ru-RU.json

@@ -2,7 +2,8 @@
   "lang": {
     "zhCN": "中文",
     "enUS": "English",
-    "ruRU": "Русский"
+    "ruRU": "Русский",
+    "arSA": "العربية"
   },
   "mobile": {
     "openNav": "Открыть меню навигации",

web/static/i18n/zh-CN.json

@@ -2,7 +2,8 @@
   "lang": {
     "zhCN": "中文",
     "enUS": "English",
-    "ruRU": "Русский"
+    "ruRU": "Русский",
+    "arSA": "العربية"
   },
   "mobile": {
     "openNav": "打开导航菜单",

web/static/js/i18n.js

@@ -29,12 +29,27 @@
         if (navLang.startsWith('ru')) {
             return 'ru-RU';
         }
+        if (navLang.startsWith('ar')) {
+            return 'ar-SA';
+        }
         if (navLang.startsWith('en')) {
             return 'en-US';
         }
         return DEFAULT_LANG;
     }
 
+    // 根据语言设置文档方向（阿拉伯语为从右到左 RTL）
+    function applyDirection(lang) {
+        try {
+            if (document && document.documentElement) {
+                var l = String(lang || DEFAULT_LANG).toLowerCase();
+                document.documentElement.dir = l.indexOf('ar') === 0 ? 'rtl' : 'ltr';
+            }
+        } catch (e) {
+            // ignore
+        }
+    }
+
     async function loadLanguageResources(lang) {
         if (loadedLangs[lang]) {
             return;
@@ -114,6 +129,9 @@
             // ignore
         }
 
+        // 更新文档方向（RTL/LTR）
+        applyDirection(i18next.language || DEFAULT_LANG);
+
         try {
             if (typeof window.syncHitlAuditBackendUI === 'function') {
                 window.syncHitlAuditBackendUI();
@@ -129,6 +147,8 @@
             label.textContent = i18next.t('lang.zhCN');
         } else if (lang.indexOf('ru') === 0) {
             label.textContent = i18next.t('lang.ruRU');
+        } else if (lang.indexOf('ar') === 0) {
+            label.textContent = i18next.t('lang.arSA');
         } else {
             label.textContent = i18next.t('lang.enUS');
         }
@@ -159,7 +179,7 @@
         const current = i18next.language || DEFAULT_LANG;
         if (lang === current) return;
         await loadLanguageResources(lang);
-        if (lang === 'ru-RU') {
+        if (lang === 'ru-RU' || lang === 'ar-SA') {
             await loadLanguageResources('en-US');
         }
         await i18next.changeLanguage(lang);
@@ -193,6 +213,7 @@
             lng: initialLang,
             fallbackLng: {
                 'ru-RU': ['en-US', 'zh-CN'],
+                'ar-SA': ['en-US', 'zh-CN'],
                 'default': [DEFAULT_LANG]
             },
             debug: false,
@@ -200,7 +221,7 @@
         });
 
         await loadLanguageResources(initialLang);
-        if (initialLang === 'ru-RU') {
+        if (initialLang === 'ru-RU' || initialLang === 'ar-SA') {
             await loadLanguageResources('en-US');
         }
         applyTranslations(document);
@@ -221,6 +242,7 @@
             const lang = String((window.__locale || (typeof i18next !== 'undefined' && i18next.language) || '')).toLowerCase();
             if (lang.indexOf('zh') === 0) return 'zh-CN';
             if (lang.indexOf('ru') === 0) return 'ru-RU';
+            if (lang.indexOf('ar') === 0) return 'ar-SA';
             return 'en-US';
         };
         window.changeLanguage = changeLanguage;

web/templates/api-docs.html

@@ -989,6 +989,7 @@
                         <div class="lang-option" data-lang="zh-CN" onclick="typeof onLanguageSelect === 'function' && onLanguageSelect('zh-CN')">中文</div>
                         <div class="lang-option" data-lang="en-US" onclick="typeof onLanguageSelect === 'function' && onLanguageSelect('en-US')">English</div>
                         <div class="lang-option" data-lang="ru-RU" onclick="typeof onLanguageSelect === 'function' && onLanguageSelect('ru-RU')">Русский</div>
+                        <div class="lang-option" data-lang="ar-SA" onclick="typeof onLanguageSelect === 'function' && onLanguageSelect('ar-SA')">العربية</div>
                     </div>
                 </div>
             </div>

web/templates/index.html

@@ -137,6 +137,7 @@
                                 <div class="lang-option" data-lang="zh-CN" onclick="onLanguageSelect('zh-CN')">中文</div>
                                 <div class="lang-option" data-lang="en-US" onclick="onLanguageSelect('en-US')">English</div>
                                 <div class="lang-option" data-lang="ru-RU" onclick="onLanguageSelect('ru-RU')">Русский</div>
+                                <div class="lang-option" data-lang="ar-SA" onclick="onLanguageSelect('ar-SA')">العربية</div>
                             </div>
                         </div>
                         <div class="notification-menu-container">