fix: harden school imports and add optional feedback diagnostics

mobile/lib/features/feedback/feedback_submission.dart

@@ -102,6 +102,7 @@ class FeedbackSubmission {
     this.contact,
     this.school,
     this.attachments = const [],
+    this.diagnosticLog,
   });
 
   factory FeedbackSubmission.problem({
@@ -110,18 +111,26 @@ class FeedbackSubmission {
     String? contact,
     List<FeedbackAttachment> attachments = const [],
     FeedbackLimits limits = FeedbackLimits.defaults,
+    Uint8List? diagnosticLog,
   }) {
     final text = message.trim();
     if (text.isEmpty) throw const FeedbackException('请填写问题或建议');
     if (text.length > 5000) throw const FeedbackException('问题或建议不能超过 5000 个字符');
     _validateContact(contact);
     validateAttachments(attachments, limits);
+    if (diagnosticLog != null &&
+        (diagnosticLog.isEmpty || diagnosticLog.length > 256 * 1024)) {
+      throw const FeedbackException('日志附件不能超过256 KB');
+    }
     return FeedbackSubmission._(
       id: id,
       kind: FeedbackKind.problem,
       message: text,
       contact: _optional(contact),
       attachments: List.unmodifiable(attachments),
+      diagnosticLog: diagnosticLog == null
+          ? null
+          : Uint8List.fromList(diagnosticLog).asUnmodifiableView(),
     );
   }
 
@@ -151,6 +160,7 @@ class FeedbackSubmission {
   final String? contact;
   final String? school;
   final List<FeedbackAttachment> attachments;
+  final Uint8List? diagnosticLog;
 
   static String newId() {
     final random = Random.secure();