feat(client): SSO 用户中心按钮 + 自动更新检查(镜像 jiu)

SSO: web_launch.openWebUserCenter → POST /v1/auth/web-ticket → 打开 app.yanmeiai.com/sso?t=…;
设置页加「用户中心(网页)」入口。自动更新: update_provider 查 /version 语义比对 + update_dialog
「发现新版本」;设置页「检查更新」由空 onTap 接上。加 url_launcher(原缺)。flutter analyze 通过。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/l10n/strings_en.dart

@@ -235,6 +235,20 @@ class StringsEn extends AppText {
   String get killSwitchSub => 'Block traffic if the link drops';
   @override
   String get checkUpdate => 'Check for updates';
+  @override
+  String get webUserCenter => 'User Center (Web)';
+  @override
+  String updateAvailableTitle(String version) => 'New version v$version available';
+  @override
+  String get updateNotesFallback => 'This update includes fixes and stability improvements.';
+  @override
+  String get updateLater => 'Later';
+  @override
+  String get updateDownload => 'Download update';
+  @override
+  String get updateUpToDate => 'You are up to date';
+  @override
+  String get updateCheckFailed => 'Update check failed, try again later';
 
   @override
   String get choosePlan => 'Choose plan';