feat: draft auto-save in compose screen

- Add Drafts table (schema v4 migration) with autoincrement id,
  accountId, replyToEmailId, to/cc/subject/body text, updatedAt
- DraftRepository interface + DraftRepositoryImpl (Drift)
- draftRepositoryProvider wired in di.dart
- ComposeScreen debounces saves (2 s after last keystroke), shows
  transient "Saved" indicator, restores the latest matching draft on
  open when no prefill fields are provided, deletes draft on send
- 6 new unit tests for DraftRepositoryImpl
- New widget test verifying draft restore behaviour
- FakeDraftRepository added to widget test helpers
- draft_repository.dart added to coverage no-code exclusion list

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

lib/data/db/database.dart

@@ -80,14 +80,27 @@ class EmailBodies extends Table {
   Set<Column> get primaryKey => {emailId};
 }
 
+/// Auto-saved compose drafts — persisted across app restarts.
+class Drafts extends Table {
+  IntColumn get id => integer().autoIncrement()();
+  TextColumn get accountId => text().nullable()();
+  /// Set for replies/reply-alls; null for new messages.
+  TextColumn get replyToEmailId => text().nullable()();
+  TextColumn get toText => text().withDefault(const Constant(''))();
+  TextColumn get ccText => text().withDefault(const Constant(''))();
+  TextColumn get subjectText => text().withDefault(const Constant(''))();
+  TextColumn get bodyText => text().withDefault(const Constant(''))();
+  DateTimeColumn get updatedAt => dateTime()();
+}
+
 // ── Database ──────────────────────────────────────────────────────────────────
 
-@DriftDatabase(tables: [Accounts, Mailboxes, Emails, EmailBodies])
+@DriftDatabase(tables: [Accounts, Mailboxes, Emails, EmailBodies, Drafts])
 class AppDatabase extends _$AppDatabase {
   AppDatabase([QueryExecutor? executor]) : super(executor ?? _openConnection());
 
   @override
-  int get schemaVersion => 3;
+  int get schemaVersion => 4;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -99,6 +112,9 @@ class AppDatabase extends _$AppDatabase {
           if (from < 3) {
             await m.addColumn(accounts, accounts.username);
           }
+          if (from < 4) {
+            await m.createTable(drafts);
+          }
         },
       );
 }

lib/data/repositories/draft_repository_impl.dart

@@ -0,0 +1,107 @@
+import 'package:drift/drift.dart';
+
+import '../../core/models/draft.dart';
+import '../../core/repositories/draft_repository.dart';
+import '../db/database.dart';
+
+class DraftRepositoryImpl implements DraftRepository {
+  DraftRepositoryImpl(this._db);
+
+  final AppDatabase _db;
+
+  @override
+  Future<SavedDraft> saveDraft({
+    int? id,
+    String? accountId,
+    String? replyToEmailId,
+    required String toText,
+    required String ccText,
+    required String subjectText,
+    required String bodyText,
+  }) async {
+    final now = DateTime.now();
+    if (id != null) {
+      await (_db.update(_db.drafts)..where((t) => t.id.equals(id))).write(
+        DraftsCompanion(
+          accountId: Value(accountId),
+          replyToEmailId: Value(replyToEmailId),
+          toText: Value(toText),
+          ccText: Value(ccText),
+          subjectText: Value(subjectText),
+          bodyText: Value(bodyText),
+          updatedAt: Value(now),
+        ),
+      );
+      return SavedDraft(
+        id: id,
+        accountId: accountId,
+        replyToEmailId: replyToEmailId,
+        toText: toText,
+        ccText: ccText,
+        subjectText: subjectText,
+        bodyText: bodyText,
+        updatedAt: now,
+      );
+    }
+
+    final newId = await _db.into(_db.drafts).insert(
+          DraftsCompanion.insert(
+            accountId: Value(accountId),
+            replyToEmailId: Value(replyToEmailId),
+            toText: Value(toText),
+            ccText: Value(ccText),
+            subjectText: Value(subjectText),
+            bodyText: Value(bodyText),
+            updatedAt: now,
+          ),
+        );
+    return SavedDraft(
+      id: newId,
+      accountId: accountId,
+      replyToEmailId: replyToEmailId,
+      toText: toText,
+      ccText: ccText,
+      subjectText: subjectText,
+      bodyText: bodyText,
+      updatedAt: now,
+    );
+  }
+
+  @override
+  Future<SavedDraft?> findDraft({String? replyToEmailId}) async {
+    final query = _db.select(_db.drafts);
+    if (replyToEmailId == null) {
+      query.where((t) => t.replyToEmailId.isNull());
+    } else {
+      query.where((t) => t.replyToEmailId.equals(replyToEmailId));
+    }
+    query.orderBy([(t) => OrderingTerm.desc(t.id)]);
+    query.limit(1);
+    final row = await query.getSingleOrNull();
+    return row == null ? null : _toModel(row);
+  }
+
+  @override
+  Future<SavedDraft?> getDraft(int id) async {
+    final row = await (_db.select(_db.drafts)
+          ..where((t) => t.id.equals(id)))
+        .getSingleOrNull();
+    return row == null ? null : _toModel(row);
+  }
+
+  @override
+  Future<void> deleteDraft(int id) async {
+    await (_db.delete(_db.drafts)..where((t) => t.id.equals(id))).go();
+  }
+
+  SavedDraft _toModel(Draft row) => SavedDraft(
+        id: row.id,
+        accountId: row.accountId,
+        replyToEmailId: row.replyToEmailId,
+        toText: row.toText,
+        ccText: row.ccText,
+        subjectText: row.subjectText,
+        bodyText: row.bodyText,
+        updatedAt: row.updatedAt,
+      );
+}