Fix school browser initialization and redirect detection; improve diagnostics

mobile/lib/services/diagnostic_log.dart

@@ -94,7 +94,45 @@ class DiagnosticLog {
     add('navigation_$phase', fields: {'school': school, 'page': page});
   }
 
+  void resourceFailure(
+    String school,
+    String? url, {
+    required String code,
+    int? status,
+    bool? mainFrame,
+  }) {
+    final uri = Uri.tryParse(url ?? '');
+    final path = uri?.path.toLowerCase() ?? '';
+    final kind = path.endsWith('.js')
+        ? 'script'
+        : path.endsWith('.css')
+        ? 'stylesheet'
+        : RegExp(r'\.(png|jpg|jpeg|gif|webp|svg|ico)$').hasMatch(path)
+        ? 'image'
+        : 'document_or_request';
+    add(
+      'school_resource_failed',
+      fields: {
+        'school': school,
+        'phase': mainFrame == null
+            ? 'unknown'
+            : mainFrame
+            ? 'main'
+            : 'subresource',
+        'page': kind,
+        'code': code,
+        'status': status,
+        'type': uri?.scheme == 'http'
+            ? 'http'
+            : uri?.scheme == 'https'
+            ? 'https'
+            : 'other',
+      },
+    );
+  }
+
   Future<void> attach(Directory directory) async {
+    // Retention and export use the same bounded, sanitized entries.
     final file = File('${directory.path}/diagnostics-recent.json');
     try {
       if (await file.exists() && await file.length() <= maxBytes) {