Implement offline mode

lib/core/services/storage_service.dart

@@ -1,7 +1,10 @@
+import 'dart:convert';
+
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:hive/hive.dart';
 import 'package:sheetless/core/models/change.dart';
 import 'package:sheetless/core/models/config.dart';
+import 'package:sheetless/core/models/sheet.dart';
 
 /// Keys for secure storage (credentials and tokens).
 enum SecureStorageKey { url, jwt, email }
@@ -29,6 +32,45 @@ class StoredAnnotation {
   }
 }
 
+/// Service for managing local storage operations.
+///
+/// Uses [FlutterSecureStorage] for sensitive data (credentials, tokens)
+/// and [Hive] for general app data (config, sheet access times, change queue,
+/// and PDF annotations).
+/// Data class for a pending annotation upload.
+class PendingAnnotationUpload {
+  final String sheetUuid;
+  final int page;
+  final String annotationsJson;
+  final DateTime lastModified;
+
+  PendingAnnotationUpload({
+    required this.sheetUuid,
+    required this.page,
+    required this.annotationsJson,
+    required this.lastModified,
+  });
+
+  Map<String, dynamic> toMap() => {
+        'sheetUuid': sheetUuid,
+        'page': page,
+        'annotationsJson': annotationsJson,
+        'lastModified': lastModified.toIso8601String(),
+      };
+
+  factory PendingAnnotationUpload.fromMap(Map<dynamic, dynamic> map) {
+    return PendingAnnotationUpload(
+      sheetUuid: map['sheetUuid'] as String,
+      page: map['page'] as int,
+      annotationsJson: map['annotationsJson'] as String,
+      lastModified: DateTime.parse(map['lastModified'] as String),
+    );
+  }
+
+  /// Unique key for deduplication (newer uploads replace older ones).
+  String get key => '${sheetUuid}_page_$page';
+}
+
 /// Service for managing local storage operations.
 ///
 /// Uses [FlutterSecureStorage] for sensitive data (credentials, tokens)
@@ -40,6 +82,8 @@ class StorageService {
   static const String _configBox = 'config';
   static const String _changeQueueBox = 'changeQueue';
   static const String _annotationsBox = 'annotations';
+  static const String _sheetsBox = 'sheets';
+  static const String _pendingAnnotationsBox = 'pendingAnnotations';
 
   late final FlutterSecureStorage _secureStorage;
 
@@ -317,4 +361,95 @@ class StorageService {
       await box.delete(key);
     }
   }
+
+  // ---------------------------------------------------------------------------
+  // Sheets Cache (Offline Support)
+  // ---------------------------------------------------------------------------
+
+  /// Reads cached sheets from local storage.
+  ///
+  /// Returns an empty list if no cached sheets exist.
+  Future<List<Sheet>> readCachedSheets() async {
+    final box = await Hive.openBox(_sheetsBox);
+    final sheetsJson = box.get('sheets');
+
+    if (sheetsJson == null) return [];
+
+    final List<dynamic> sheetsList = jsonDecode(sheetsJson as String);
+    return sheetsList
+        .map((json) => Sheet.fromJson(json as Map<String, dynamic>))
+        .toList();
+  }
+
+  /// Caches the sheets list to local storage.
+  Future<void> writeCachedSheets(List<Sheet> sheets) async {
+    final box = await Hive.openBox(_sheetsBox);
+    final sheetsJson = jsonEncode(sheets.map((s) => s.toJson()).toList());
+    await box.put('sheets', sheetsJson);
+  }
+
+  // ---------------------------------------------------------------------------
+  // Pending Annotation Uploads (Offline Support)
+  // ---------------------------------------------------------------------------
+
+  /// Adds or updates a pending annotation upload.
+  ///
+  /// If an upload for the same sheet/page already exists, it will be replaced
+  /// with the newer version.
+  Future<void> writePendingAnnotationUpload(
+    PendingAnnotationUpload upload,
+  ) async {
+    final box = await Hive.openBox(_pendingAnnotationsBox);
+    await box.put(upload.key, upload.toMap());
+  }
+
+  /// Reads all pending annotation uploads.
+  Future<List<PendingAnnotationUpload>> readPendingAnnotationUploads() async {
+    final box = await Hive.openBox(_pendingAnnotationsBox);
+    final uploads = <PendingAnnotationUpload>[];
+
+    for (final value in box.values) {
+      uploads.add(PendingAnnotationUpload.fromMap(value as Map));
+    }
+
+    return uploads;
+  }
+
+  /// Removes a pending annotation upload after successful sync.
+  Future<void> deletePendingAnnotationUpload(String key) async {
+    final box = await Hive.openBox(_pendingAnnotationsBox);
+    await box.delete(key);
+  }
+
+  /// Checks if there are any pending annotation uploads.
+  Future<bool> hasPendingAnnotationUploads() async {
+    final box = await Hive.openBox(_pendingAnnotationsBox);
+    return box.isNotEmpty;
+  }
+
+  // ---------------------------------------------------------------------------
+  // Change Queue Enhancements
+  // ---------------------------------------------------------------------------
+
+  /// Returns the number of pending changes.
+  Future<int> getChangeQueueLength() async {
+    final box = await Hive.openBox(_changeQueueBox);
+    return box.length;
+  }
+
+  /// Clears all pending changes.
+  ///
+  /// Use with caution - only call after all changes are synced.
+  Future<void> clearChangeQueue() async {
+    final box = await Hive.openBox(_changeQueueBox);
+    await box.clear();
+  }
+
+  /// Gets all changes as a list (for batch upload).
+  Future<List<Change>> readChangeList() async {
+    final box = await Hive.openBox(_changeQueueBox);
+    return box.values
+        .map((map) => Change.fromMap(map as Map<dynamic, dynamic>))
+        .toList();
+  }
 }