feat(auth): account settings page + i18n

- Port account-settings-page.tsx (change password, change email, logout)
- Wire into settings-dialog.tsx as new "account" section with UserIcon,
  rendered first in the section list
- Add i18n keys:
  - en-US/zh-CN: settings.sections.account ("Account" / "账号")
  - en-US/zh-CN: button.logout ("Log out" / "退出登录")
  - types.ts: matching type declarations

frontend/src/core/i18n/locales/en-US.ts

@@ -236,6 +236,7 @@ export const enUS: Translations = {
     reportIssue: "Report a issue",
     contactUs: "Contact us",
     about: "About DeerFlow",
+    logout: "Log out",
   },
 
   // Conversation
@@ -320,6 +321,7 @@ export const enUS: Translations = {
     title: "Settings",
     description: "Adjust how DeerFlow looks and behaves for you.",
     sections: {
+      account: "Account",
       appearance: "Appearance",
       memory: "Memory",
       tools: "Tools",

frontend/src/core/i18n/locales/types.ts

@@ -168,6 +168,7 @@ export interface Translations {
     reportIssue: string;
     contactUs: string;
     about: string;
+    logout: string;
   };
 
   // Conversation
@@ -250,6 +251,7 @@ export interface Translations {
     title: string;
     description: string;
     sections: {
+      account: string;
       appearance: string;
       memory: string;
       tools: string;

frontend/src/core/i18n/locales/zh-CN.ts

@@ -224,6 +224,7 @@ export const zhCN: Translations = {
     reportIssue: "报告问题",
     contactUs: "联系我们",
     about: "关于 DeerFlow",
+    logout: "退出登录",
   },
 
   // Conversation
@@ -305,6 +306,7 @@ export const zhCN: Translations = {
     title: "设置",
     description: "根据你的偏好调整 DeerFlow 的界面和行为。",
     sections: {
+      account: "账号",
       appearance: "外观",
       memory: "记忆",
       tools: "工具",