feat(email): show nested MIME structure in email detail screen (#88)

Adds a MimePart tree model, parses it from the IMAP BODYSTRUCTURE
when fetching the email body, caches it in a new mime_tree_json column
(schema v28), and exposes a 'Show Mail Structure' overflow menu item
that renders the indented tree (content-type, filename, size, encoding)
in an AlertDialog alongside the existing headers dialog.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/data/db/database.dart

@@ -107,6 +107,8 @@ class EmailBodies extends Table {
   DateTimeColumn get cachedAt => dateTime().nullable()();
   // Added in schema v20: raw or parsed headers
   TextColumn get headersJson => text().nullable()();
+  // Added in schema v28: serialised MimePart tree (JSON)
+  TextColumn get mimeTreeJson => text().nullable()();
 
   @override
   Set<Column> get primaryKey => {emailId};
@@ -277,7 +279,7 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase([QueryExecutor? executor]) : super(executor ?? _openConnection());
 
   @override
-  int get schemaVersion => 27;
+  int get schemaVersion => 28;
 
   Future<void> _createEmailFts() async {
     await customStatement('''
@@ -503,6 +505,9 @@ class AppDatabase extends _$AppDatabase {
           if (from < 27) {
             await m.createTable(searchHistoryEntries);
           }
+          if (from < 28) {
+            await m.addColumn(emailBodies, emailBodies.mimeTreeJson);
+          }
         },
       );
 }

lib/data/repositories/email_repository_impl.dart

@@ -262,6 +262,8 @@ class EmailRepositoryImpl implements EmailRepository {
             .toList(),
       );
 
+      final mimeTreeJson = _buildMimeTreeJson(msg);
+
       await _db.into(_db.emailBodies).insertOnConflictUpdate(
             EmailBodiesCompanion.insert(
               emailId: emailId,
@@ -269,6 +271,7 @@ class EmailRepositoryImpl implements EmailRepository {
               htmlBody: Value(htmlBody),
               attachmentsJson: Value(attachmentsJson),
               headersJson: Value(headersJson),
+              mimeTreeJson: Value(mimeTreeJson),
               cachedAt: Value(DateTime.now()),
             ),
           );
@@ -278,6 +281,7 @@ class EmailRepositoryImpl implements EmailRepository {
         htmlBody: htmlBody,
         attachments: _parseAttachments(attachmentsJson),
         headers: _parseHeaders(headersJson),
+        mimeTree: _parseMimeTree(mimeTreeJson),
       );
     } finally {
       await client.logout();
@@ -2882,6 +2886,27 @@ class EmailRepositoryImpl implements EmailRepository {
         htmlBody: row.htmlBody,
         attachments: _parseAttachments(row.attachmentsJson),
         headers: _parseHeaders(row.headersJson),
+        mimeTree: _parseMimeTree(row.mimeTreeJson),
+      );
+
+  model.MimePart? _parseMimeTree(String? jsonStr) {
+    if (jsonStr == null || jsonStr.isEmpty) return null;
+    try {
+      return _mimePartFromJson(jsonDecode(jsonStr) as Map<String, dynamic>);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  model.MimePart _mimePartFromJson(Map<String, dynamic> m) => model.MimePart(
+        contentType: m['contentType'] as String? ?? 'application/octet-stream',
+        filename: m['filename'] as String?,
+        size: m['size'] as int?,
+        encoding: m['encoding'] as String?,
+        children: ((m['children'] as List<dynamic>?) ?? [])
+            .cast<Map<String, dynamic>>()
+            .map(_mimePartFromJson)
+            .toList(),
       );
 
   List<model.EmailHeader> _parseHeaders(String? jsonStr) {
@@ -2973,3 +2998,23 @@ class EmailRepositoryImpl implements EmailRepository {
     }
   }
 }
+
+/// Recursively converts an [imap.MimePart] into a JSON-serialisable map.
+Map<String, dynamic> _mimePartToJson(imap.MimePart part) {
+  final ct = part.getHeaderContentType();
+  final disposition = part.getHeaderContentDisposition();
+  final rawEncoding =
+      part.getHeader('content-transfer-encoding')?.firstOrNull?.value;
+  final encoding = rawEncoding?.split(';').first.trim().toLowerCase();
+  return {
+    'contentType': ct?.mediaType.text ?? 'application/octet-stream',
+    'filename': disposition?.filename ?? ct?.parameters['name'],
+    'size': disposition?.size,
+    'encoding': encoding,
+    'children': (part.parts ?? []).map(_mimePartToJson).toList(),
+  };
+}
+
+/// Builds a JSON string representing the MIME tree of [msg].
+String _buildMimeTreeJson(imap.MimeMessage msg) =>
+    jsonEncode(_mimePartToJson(msg));