feat: add app update checks and standalone release publishing panel

mobile/lib/features/profile/profile_page.dart

@@ -24,6 +24,8 @@ class ProfilePage extends StatelessWidget {
     this.logoutLabel = '退出学校登录',
     this.onUpdateTerm,
     this.feedbackSender = const UnconfiguredFeedbackSender(),
+    this.onCheckUpdates,
+    this.checkingUpdates = false,
   });
   final AppController controller;
   final VoidCallback onLogin;
@@ -32,6 +34,8 @@ class ProfilePage extends StatelessWidget {
   final VoidCallback? onUpdateTerm;
   final String authStatus, logoutLabel;
   final FeedbackSender feedbackSender;
+  final VoidCallback? onCheckUpdates;
+  final bool checkingUpdates;
   Future<void> _import(BuildContext context) async {
     try {
       final file = await FilePicker.pickFile(
@@ -231,6 +235,32 @@ class ProfilePage extends StatelessWidget {
                   ),
                 ),
               ),
+              Divider(height: 1, color: ink.withValues(alpha: 0.06)),
+              ListTile(
+                key: const Key('check-app-update'),
+                contentPadding: EdgeInsets.zero,
+                leading: const Icon(
+                  Icons.system_update_rounded,
+                  color: river,
+                  size: 23,
+                ),
+                title: const Text(
+                  '检查更新',
+                  style: TextStyle(fontSize: 14, fontWeight: FontWeight.w600),
+                ),
+                subtitle: Text(
+                  checkingUpdates ? '正在检查…' : '当前版本 $appVersion',
+                  style: const TextStyle(fontSize: 11),
+                ),
+                trailing: checkingUpdates
+                    ? const SizedBox(
+                        width: 18,
+                        height: 18,
+                        child: CircularProgressIndicator(strokeWidth: 2),
+                      )
+                    : const Icon(Icons.chevron_right_rounded),
+                onTap: checkingUpdates ? null : onCheckUpdates,
+              ),
             ],
           ),
         ),

mobile/lib/features/updates/update_prompt.dart

@@ -0,0 +1,77 @@
+import 'package:flutter/material.dart';
+
+import '../../app/app_version.dart';
+import '../../services/app_update_service.dart';
+
+Future<void> showUpdatePrompt(
+  BuildContext context,
+  AppRelease release, {
+  UpdateDownloadOpener openDownload = openUpdateInBrowser,
+}) async {
+  final confirmed = await showDialog<bool>(
+    context: context,
+    builder: (context) => AlertDialog(
+      title: Text('发现新版本 ${release.version}'),
+      content: SizedBox(
+        width: 360,
+        child: SingleChildScrollView(
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children: [
+              const Text('当前版本 $appVersion', style: TextStyle(fontSize: 12)),
+              const SizedBox(height: 16),
+              Text(release.notes, style: const TextStyle(height: 1.6)),
+              const SizedBox(height: 16),
+              const Text(
+                '将打开浏览器下载，安装时请保留现有应用和课表。',
+                style: TextStyle(fontSize: 12),
+              ),
+            ],
+          ),
+        ),
+      ),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.pop(context, false),
+          child: const Text('稍后'),
+        ),
+        FilledButton(
+          onPressed: () => Navigator.pop(context, true),
+          child: const Text('立即更新'),
+        ),
+      ],
+    ),
+  );
+  if (confirmed != true || !context.mounted) return;
+  bool opened;
+  try {
+    opened = await openDownload(release.downloadUri);
+  } catch (_) {
+    opened = false;
+  }
+  if (!opened && context.mounted) {
+    await showDialog<void>(
+      context: context,
+      builder: (context) => AlertDialog(
+        title: const Text('未能打开浏览器'),
+        content: SingleChildScrollView(
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              const Text('可以复制下方链接到浏览器下载：'),
+              const SizedBox(height: 12),
+              SelectableText(release.downloadUri.toString()),
+            ],
+          ),
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(context),
+            child: const Text('知道了'),
+          ),
+        ],
+      ),
+    );
+  }
+}