fix(ui): localize language selector and validate stored locale

- Add translation keys for language selector label and language names
   - Update all locale files (en, pt-BR, zh-CN, zh-TW) with:
     - overview.access.language key for selector label
     - languages.* keys for language display names
   - Localize language selector in overview.ts to react to locale changes
   - Add validation for stored locale in app.ts to prevent invalid values
     from causing silent failures in setLocale

   Fixes issues identified in code review:
   - Unlocalized language selector inconsistency
   - Settings locale type drift risk

ui/src/i18n/locales/en.ts

@@ -59,6 +59,7 @@ export const en: TranslationMap = {
       token: "Gateway Token",
       password: "Password (not stored)",
       sessionKey: "Default Session Key",
+      language: "Language",
       connectHint: "Click Connect to apply connection changes.",
     },
     snapshot: {
@@ -104,4 +105,10 @@ export const en: TranslationMap = {
     focusToggle: "Toggle focus mode (hide sidebar + page header)",
     onboardingDisabled: "Disabled during onboarding",
   },
+  languages: {
+    en: "English",
+    zhCN: "简体中文 (Simplified Chinese)",
+    zhTW: "繁體中文 (Traditional Chinese)",
+    ptBR: "Português (Brazilian Portuguese)",
+  },
 };

ui/src/i18n/locales/pt-BR.ts

@@ -59,6 +59,7 @@ export const pt_BR: TranslationMap = {
       token: "Token do Gateway",
       password: "Senha (não armazenada)",
       sessionKey: "Chave de Sessão Padrão",
+      language: "Idioma",
       connectHint: "Clique em Conectar para aplicar as alterações de conexão.",
     },
     snapshot: {
@@ -104,4 +105,10 @@ export const pt_BR: TranslationMap = {
     focusToggle: "Alternar modo de foco (ocultar barra lateral + cabeçalho da página)",
     onboardingDisabled: "Desativado durante a integração",
   },
+  languages: {
+    en: "English",
+    zhCN: "简体中文 (Chinês Simplificado)",
+    zhTW: "繁體中文 (Chinês Tradicional)",
+    ptBR: "Português (Português Brasileiro)",
+  },
 };

ui/src/i18n/locales/zh-CN.ts

@@ -59,6 +59,7 @@ export const zh_CN: TranslationMap = {
       token: "网关令牌",
       password: "密码 (不存储)",
       sessionKey: "默认会话密钥",
+      language: "语言",
       connectHint: "点击连接以应用连接更改。",
     },
     snapshot: {
@@ -104,4 +105,10 @@ export const zh_CN: TranslationMap = {
     focusToggle: "切换专注模式 (隐藏侧边栏 + 页面页眉)",
     onboardingDisabled: "引导期间禁用",
   },
+  languages: {
+    en: "English",
+    zhCN: "简体中文 (简体中文)",
+    zhTW: "繁體中文 (繁体中文)",
+    ptBR: "Português (巴西葡萄牙语)",
+  },
 };

ui/src/i18n/locales/zh-TW.ts

@@ -55,6 +55,7 @@ export const zh_TW: TranslationMap = {
       token: "網關令牌",
       password: "密碼 (不存儲)",
       sessionKey: "默認會話密鑰",
+      language: "語言",
       connectHint: "點擊連接以應用連接更改。",
     },
     snapshot: {
@@ -100,4 +101,10 @@ export const zh_TW: TranslationMap = {
     focusToggle: "切換專注模式 (隱藏側邊欄 + 頁面頁眉)",
     onboardingDisabled: "引導期間禁用",
   },
+  languages: {
+    en: "English",
+    zhCN: "简体中文 (簡體中文)",
+    zhTW: "繁體中文 (繁體中文)",
+    ptBR: "Português (巴西葡萄牙語)",
+  },
 };

ui/src/ui/views/overview.ts

@@ -165,7 +165,7 @@ export function renderOverview(props: OverviewProps) {
             />
           </label>
           <label class="field">
-            <span>Language</span>
+            <span>${t("overview.access.language")}</span>
             <select
               .value=${currentLocale}
               @change=${(e: Event) => {
@@ -174,10 +174,10 @@ export function renderOverview(props: OverviewProps) {
                 props.onSettingsChange({ ...props.settings, locale: v });
               }}
             >
-              <option value="en">English</option>
-              <option value="zh-CN">简体中文 (Simplified Chinese)</option>
-              <option value="zh-TW">繁體中文 (Traditional Chinese)</option>
-              <option value="pt-BR">Português (Brazilian Portuguese)</option>
+              <option value="en">${t("languages.en")}</option>
+              <option value="zh-CN">${t("languages.zhCN")}</option>
+              <option value="zh-TW">${t("languages.zhTW")}</option>
+              <option value="pt-BR">${t("languages.ptBR")}</option>
             </select>
           </label>
         </div>