feat: refine AI navigation and accessibility recovery

frontend/lib/features/home/pages/main_shell.dart

@@ -25,14 +25,6 @@ class _MainShellState extends State<MainShell> {
     return AnimatedBuilder(
       animation: SessionStore.instance,
       builder: (context, _) {
-        final session = SessionStore.instance;
-        final aiEnabled = session.aiEnabled;
-        final showAiEntry = session.isGuest || aiEnabled;
-        if (!showAiEntry && widget.shell.currentIndex == 2) {
-          WidgetsBinding.instance.addPostFrameCallback((_) {
-            if (mounted) widget.shell.goBranch(0);
-          });
-        }
         return Scaffold(
           body: AnimatedSwitcher(
             duration: const Duration(milliseconds: 200),
@@ -58,12 +50,11 @@ class _MainShellState extends State<MainShell> {
                     _tab('明细', AppIcons.home, 0),
                     _tab('统计', AppIcons.chart, 1),
                     SizedBox(width: 72, child: _fab()),
-                    if (showAiEntry)
-                      ValueListenableBuilder<CompanionDisplay>(
-                        valueListenable: PublicConfigApi.companionNotifier,
-                        builder: (_, companion, __) =>
-                            _tab(companion.name, AppIcons.chat, 2),
-                      ),
+                    ValueListenableBuilder<CompanionDisplay>(
+                      valueListenable: PublicConfigApi.companionNotifier,
+                      builder: (_, companion, __) =>
+                          _tab(companion.name, AppIcons.chat, 2),
+                    ),
                     _tab('我的', AppIcons.user, 3),
                   ],
                 ),