feat: refine AI navigation and accessibility recovery

frontend/lib/shared/widgets/ai_access_gate.dart

@@ -6,12 +6,13 @@ import 'package:miaoji_zhang/shared/theme/app_theme.dart';
 import 'package:miaoji_zhang/shared/widgets/app_controls.dart';
 import 'package:miaoji_zhang/shared/widgets/app_icons.dart';
 
-enum AiAccessState { guest, reauthenticate, cloudDisabled }
+enum AiAccessState { guest, reauthenticate, aiDisabled, cloudDisabled }
 
 AiAccessState? currentAiAccessState() {
   final session = SessionStore.instance;
   if (session.isGuest) return AiAccessState.guest;
   if (session.needsReauth) return AiAccessState.reauthenticate;
+  if (!session.aiEnabled) return AiAccessState.aiDisabled;
   if (!session.cloudSyncEnabled) return AiAccessState.cloudDisabled;
   return null;
 }
@@ -32,23 +33,27 @@ class _AiAccessGateState extends State<AiAccessGate> {
   String get _title => switch (widget.state) {
     AiAccessState.guest => '登录后使用 AI 助手',
     AiAccessState.reauthenticate => '登录状态已过期',
+    AiAccessState.aiDisabled => 'AI 功能已关闭',
     AiAccessState.cloudDisabled => 'AI 功能需要云连接',
   };
 
   String get _message => switch (widget.state) {
     AiAccessState.guest => '游客账单会继续安全保存在本机。登录后即可使用 AI 聊天、语音解析和图片识别。',
     AiAccessState.reauthenticate => '本地记账不受影响。重新登录后可以继续使用 AI 和云同步。',
+    AiAccessState.aiDisabled => '当前账号暂未开通 AI，手动记账和统计不受影响。',
     AiAccessState.cloudDisabled => '当前账号仅使用本地数据。开启云同步后才能发送 AI 消息。',
   };
 
-  String get _actionLabel => switch (widget.state) {
+  String? get _actionLabel => switch (widget.state) {
     AiAccessState.guest => '登录后使用',
     AiAccessState.reauthenticate => '重新登录',
+    AiAccessState.aiDisabled => null,
     AiAccessState.cloudDisabled => '开启云同步',
   };
 
   Future<void> _act() async {
     if (_busy) return;
+    if (widget.state == AiAccessState.aiDisabled) return;
     if (widget.onAction != null) {
       await widget.onAction!();
       return;
@@ -115,15 +120,17 @@ class _AiAccessGateState extends State<AiAccessGate> {
                       height: 1.6,
                     ),
                   ),
-                  SizedBox(height: 22),
-                  SizedBox(
-                    width: double.infinity,
-                    child: JzActionButton(
-                      label: _actionLabel,
-                      loading: _busy,
-                      onPressed: _busy ? null : _act,
+                  if (_actionLabel case final label?) ...[
+                    SizedBox(height: 22),
+                    SizedBox(
+                      width: double.infinity,
+                      child: JzActionButton(
+                        label: label,
+                        loading: _busy,
+                        onPressed: _busy ? null : _act,
+                      ),
                     ),
-                  ),
+                  ],
                   if (widget.state == AiAccessState.guest) ...[
                     SizedBox(height: 12),
                     Text(