feat(client/i18n): 新增 日/韩/俄/西 4 语,默认语种改英文

- AppLang enum 扩为 zh/en/ja/ko/ru/es;新增 strings_{ja,ko,ru,es}.dart
  (各 161 条,照 strings_en.dart 结构逐条翻译,无红线词,scan-redline 过)
- app_providers: localeProvider 默认 AppLang.en(国际化默认);appTextProvider
  三元改 switch 全覆盖 6 语
- AppLang.nativeLabel 扩展(各语言本地名);settings/account 两处 _LangSwitch
  段控改下拉(6 语横排会挤)
- flutter analyze lib 仅剩 2 个既有 withOpacity deprecation info

注:节点/套餐名、错误消息等 ~11 处数据驱动的 zh/en 三元暂回退 en(非崩溃,
后续可扩多语字段)。localeProvider 暂无持久化(沿用原设计,后续可加)。

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

client/lib/screens/account_page.dart

@@ -351,31 +351,37 @@ class _LangSwitch extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final c = context.pangolin;
-    Widget seg(AppLang v, String label) {
-      final active = lang == v;
-      return GestureDetector(
-        onTap: () => onChange(v),
-        child: Container(
-          padding: const EdgeInsets.symmetric(horizontal: 13, vertical: 5),
-          decoration: BoxDecoration(
-            color: active ? c.accent : Colors.transparent,
-            borderRadius: BorderRadius.circular(PangolinRadius.full),
+    // 6 种语言用下拉(段控横排会挤)。
+    return PopupMenuButton<AppLang>(
+      initialValue: lang,
+      onSelected: onChange,
+      color: c.surface,
+      elevation: 8,
+      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(PangolinRadius.md)),
+      itemBuilder: (_) => [
+        for (final l in AppLang.values)
+          PopupMenuItem<AppLang>(
+            value: l,
+            height: 42,
+            child: Text(l.nativeLabel,
+                style: PangolinText.caption.copyWith(
+                    color: l == lang ? c.accent : c.fg1,
+                    fontWeight: l == lang ? FontWeight.w700 : FontWeight.w500,
+                    fontSize: 13)),
           ),
-          child: Text(label,
-              style: PangolinText.caption.copyWith(
-                  color: active ? c.fgOnAccent : c.fg3, fontWeight: FontWeight.w700, fontSize: 12.5)),
-        ),
-      );
-    }
-
-    return Container(
-      padding: const EdgeInsets.all(3),
-      decoration: BoxDecoration(color: c.bgSubtle, borderRadius: BorderRadius.circular(PangolinRadius.full)),
-      child: Row(mainAxisSize: MainAxisSize.min, children: [
-        seg(AppLang.zh, '中文'),
-        const SizedBox(width: 2),
-        seg(AppLang.en, 'EN'),
-      ]),
+      ],
+      child: Container(
+        padding: const EdgeInsets.symmetric(horizontal: 13, vertical: 6),
+        decoration:
+            BoxDecoration(color: c.bgSubtle, borderRadius: BorderRadius.circular(PangolinRadius.full)),
+        child: Row(mainAxisSize: MainAxisSize.min, children: [
+          Text(lang.nativeLabel,
+              style: PangolinText.caption
+                  .copyWith(color: c.fg2, fontWeight: FontWeight.w700, fontSize: 12.5)),
+          const SizedBox(width: 4),
+          Icon(PangolinIcons.chevronDown, size: 14, color: c.fg3),
+        ]),
+      ),
     );
   }
 }

client/lib/screens/settings_page.dart

@@ -148,27 +148,37 @@ class _LangSwitch extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final c = context.pangolin;
-    Widget seg(AppLang v, String label) {
-      final on = lang == v;
-      return GestureDetector(
-        onTap: () => onPick(v),
-        child: Container(
-          padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 4),
-          decoration: BoxDecoration(
-            color: on ? c.accent : Colors.transparent,
-            borderRadius: BorderRadius.circular(PangolinRadius.full),
+    // 6 种语言用下拉(段控横排会挤)。当前语言以本地名 + 下拉箭头展示。
+    return PopupMenuButton<AppLang>(
+      initialValue: lang,
+      onSelected: onPick,
+      color: c.surface,
+      elevation: 8,
+      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(PangolinRadius.md)),
+      itemBuilder: (_) => [
+        for (final l in AppLang.values)
+          PopupMenuItem<AppLang>(
+            value: l,
+            height: 42,
+            child: Text(l.nativeLabel,
+                style: PangolinText.caption.copyWith(
+                    color: l == lang ? c.accent : c.fg1,
+                    fontWeight: l == lang ? FontWeight.w700 : FontWeight.w500,
+                    fontSize: 13)),
           ),
-          child: Text(label,
-              style: PangolinText.caption.copyWith(
-                  color: on ? c.fgOnAccent : c.fg3, fontWeight: FontWeight.w700, fontSize: 12)),
-        ),
-      );
-    }
-
-    return Container(
-      padding: const EdgeInsets.all(3),
-      decoration: BoxDecoration(color: c.bgSubtle, borderRadius: BorderRadius.circular(PangolinRadius.full)),
-      child: Row(mainAxisSize: MainAxisSize.min, children: [seg(AppLang.zh, '中文'), seg(AppLang.en, 'EN')]),
+      ],
+      child: Container(
+        padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 6),
+        decoration:
+            BoxDecoration(color: c.bgSubtle, borderRadius: BorderRadius.circular(PangolinRadius.full)),
+        child: Row(mainAxisSize: MainAxisSize.min, children: [
+          Text(lang.nativeLabel,
+              style: PangolinText.caption
+                  .copyWith(color: c.fg2, fontWeight: FontWeight.w700, fontSize: 12)),
+          const SizedBox(width: 4),
+          Icon(PangolinIcons.chevronDown, size: 14, color: c.fg3),
+        ]),
+      ),
     );
   }
 }