fix(client): 账号菜单改 showMenu 等宽触发器 + 向上弹（修菜单溢出盖内容）

上一版用 PopupMenuButton width:218 + offset:-12 反而把菜单外扩、右缘越过侧栏
盖住内容区。改为 GestureDetector + showMenu，菜单宽度 = 用户区实测宽度（自动与
侧栏内容等宽、绝不溢出），底部空间不足 showMenu 自动向上弹、不遮触发器。
加 user_menu_width_test：点开后断言菜单宽≈194、右缘≤218（弹出态回归）。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TSKEiHsvauyxYUW2itzUXX

client/lib/screens/shell/app_shell.dart

@@ -84,6 +84,34 @@ class _AppShellState extends ConsumerState<AppShell> {
   bool _forceDialogShown = false;
   bool _licenseDialogShown = false;
   final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
+  // 侧栏底部用户区，用于实测宽度让弹出菜单等宽 + 定位（向上弹，不溢出内容区）
+  final GlobalKey _sideUserKey = GlobalKey();
+
+  /// 打开账号菜单（个人设置 / 退出登录）。菜单宽度 = 用户区实测宽度（自动与侧栏
+  /// 内容等宽、绝不溢出到内容区）；底部空间不足时 showMenu 自动向上弹。
+  void _openUserMenu(BuildContext context) {
+    final t = context.tokens;
+    final rb = _sideUserKey.currentContext?.findRenderObject() as RenderBox?;
+    final overlay =
+        Overlay.of(context).context.findRenderObject() as RenderBox?;
+    if (rb == null || overlay == null) return;
+    final topLeft = rb.localToGlobal(Offset.zero, ancestor: overlay);
+    final rect = topLeft & rb.size;
+    showMenu<String>(
+      context: context,
+      position: RelativeRect.fromRect(rect, Offset.zero & overlay.size),
+      constraints: BoxConstraints.tightFor(width: rb.size.width),
+      color: t.menuUserBg,
+      elevation: 8,
+      shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(6)),
+      items: [
+        _userMenuItem('profile', Icons.manage_accounts_outlined, '个人设置', t),
+        _userMenuItem('logout', Icons.logout, '退出登录', t),
+      ],
+    ).then((v) {
+      if (v == 'logout') _logout();
+    });
+  }
 
   /// 切换到第 index 个分支；再次点当前栏目则回到该分支初始页。
   void _goBranch(int index) {
@@ -286,24 +314,11 @@ class _AppShellState extends ConsumerState<AppShell> {
         ),
         Padding(
           padding: const EdgeInsets.only(bottom: 8),
-          child: PopupMenuButton<String>(
-            tooltip: '账号菜单',
-            position: PopupMenuPosition.over,
-            // 菜单与侧栏等宽（218），左对齐到侧栏左缘（抵消侧栏 12 内边距）
-            offset: const Offset(-12, -8),
-            constraints: const BoxConstraints.tightFor(width: 218),
-            shape:
-                RoundedRectangleBorder(borderRadius: BorderRadius.circular(6)),
-            color: t.menuUserBg,
-            elevation: 8,
-            onSelected: (v) {
-              if (v == 'logout') _logout();
-            },
-            itemBuilder: (_) => [
-              _userMenuItem('profile', Icons.manage_accounts_outlined, '个人设置', t),
-              _userMenuItem('logout', Icons.logout, '退出登录', t),
-            ],
+          child: GestureDetector(
+            behavior: HitTestBehavior.opaque,
+            onTap: () => _openUserMenu(context),
             child: Container(
+              key: _sideUserKey,
               padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 9),
               decoration: BoxDecoration(
                 borderRadius: BorderRadius.circular(6),