refactor: apply SOLID principles and introduce domain layer

Separate concerns by introducing a Pictogram value object and Board
entity, extracting business logic from the screen into dedicated
services (BoardShareService, RecommendationService), and reducing
the screen to presentation-only responsibilities.

- Add models/pictogram.dart (value object with parsing and identity)
- Add models/board.dart (entity with business rules and ChangeNotifier)
- Extract services/board_share_service.dart from screen
- Extract services/recommendation_service.dart from utils
- Simplify pictogram_utils.dart to delegate to models and services
- Add unit tests for Pictogram, Board and RecommendationService
- Update README with new project structure and features

Author: Endika

README.md

@@ -36,8 +36,9 @@ Pictograms are universally easy to identify, making communication simple and int
 
 - **Pictogram keyboard** with categorized icons (descriptive, people, prepositions, determiners, nouns, verbs)
 - **Visual board** to compose messages by selecting pictograms
+- **Smart recommendations** — contextual next-word suggestions based on communication patterns
 - **Share** the board as a 1080×1080 image with text description
-- **Installable PWA** — add to home screen on Android and iOS
+- **Offline-first PWA** — works without internet after the first visit, installable on any device
 - **Multi-language support**: English, Spanish, Catalan, Basque, French, Galician, Portuguese, Valencian
 - **Accessibility-focused** design with semantic labels for screen readers
 
@@ -70,23 +71,29 @@ flutter test
 
 ```
 lib/
-├── main.dart                     # App entry point
+├── main.dart                          # App entry point
+├── models/
+│   ├── pictogram.dart                 # Pictogram value object
+│   └── board.dart                     # Board entity with business rules
 ├── data/
-│   └── pictogram_data.dart       # Pictogram categories and icon data
+│   ├── pictogram_data.dart            # Pictogram categories and icon data
+│   └── pictogram_recommendations.dart # Next-word suggestion graph
 ├── screens/
-│   └── pictotap_screen.dart      # Main screen with board and state
+│   └── pictotap_screen.dart           # Main screen (presentation only)
 ├── services/
-│   ├── image_saver.dart          # Platform export selector
-│   ├── image_saver_native.dart   # Native image sharing (Android/iOS)
-│   ├── image_saver_web.dart      # Web image download / Web Share API
-│   └── image_saver_stub.dart     # Stub for unsupported platforms
+│   ├── image_saver.dart               # Platform export selector
+│   ├── image_saver_native.dart        # Native image sharing (Android/iOS)
+│   ├── image_saver_web.dart           # Web image download / Web Share API
+│   ├── image_saver_stub.dart          # Stub for unsupported platforms
+│   ├── board_share_service.dart       # Board capture and share pipeline
+│   └── recommendation_service.dart    # Contextual pictogram suggestions
 ├── utils/
-│   └── pictogram_utils.dart      # Icon utility functions
+│   └── pictogram_utils.dart           # Convenience wrappers
 ├── widgets/
-│   ├── board_empty_hint.dart     # Empty board hint animation
-│   ├── pictogram_icon.dart       # Pictogram icon widget
-│   └── pictogram_keyboard.dart   # Keyboard widget with categories
-└── l10n/                         # Localization (ARB files)
+│   ├── board_empty_hint.dart          # Empty board hint animation
+│   ├── pictogram_icon.dart            # Pictogram icon widget
+│   └── pictogram_keyboard.dart        # Keyboard widget with categories
+└── l10n/                              # Localization (ARB files)
 ```
 
 ## Team

lib/models/board.dart

@@ -0,0 +1,41 @@
+import 'package:flutter/foundation.dart';
+import 'package:pictotap/data/pictogram_data.dart';
+import 'package:pictotap/models/pictogram.dart';
+
+class Board extends ChangeNotifier {
+  final List<Pictogram> _icons = [];
+  final int maxIcons;
+
+  Board({this.maxIcons = defaultBoardMaxIcons});
+
+  List<Pictogram> get icons => List.unmodifiable(_icons);
+  int get length => _icons.length;
+  bool get isEmpty => _icons.isEmpty;
+  bool get isFull => _icons.length >= maxIcons;
+
+  List<String> get iconIds => _icons.map((p) => p.id).toList();
+
+  String get shareText => _icons.map((p) => p.displayName).join(' ');
+
+  bool add(Pictogram pictogram) {
+    if (isFull) return true;
+    _icons.add(pictogram);
+    notifyListeners();
+    return _icons.length >= maxIcons;
+  }
+
+  bool addSpace() => add(Pictogram.space);
+
+  Pictogram? removeLast() {
+    if (_icons.isEmpty) return null;
+    final removed = _icons.removeLast();
+    notifyListeners();
+    return removed;
+  }
+
+  void clear() {
+    if (_icons.isEmpty) return;
+    _icons.clear();
+    notifyListeners();
+  }
+}

lib/models/pictogram.dart

@@ -0,0 +1,53 @@
+import 'package:pictotap/data/pictogram_data.dart';
+
+const List<String> _knownCategories = [
+  'descriptive',
+  'people',
+  'prepositions',
+  'some',
+  'substantive',
+  'verbs',
+];
+
+class Pictogram {
+  final String category;
+  final String name;
+
+  const Pictogram._({required this.category, required this.name});
+
+  static const Pictogram space = Pictogram._(category: '', name: spaceIcon);
+
+  factory Pictogram.fromId(String id) {
+    if (id == spaceIcon) return space;
+    final sep = id.indexOf(':');
+    if (sep <= 0) {
+      return Pictogram._(category: '', name: id);
+    }
+    return Pictogram._(
+      category: id.substring(0, sep),
+      name: id.substring(sep + 1),
+    );
+  }
+
+  String get id => isSpace ? spaceIcon : '$category:$name';
+
+  bool get isSpace => identical(this, space) || name == spaceIcon;
+
+  bool get isAsset =>
+      category.isNotEmpty && _knownCategories.contains(category);
+
+  String get displayName => isSpace ? ' ' : name;
+
+  String? get assetPath =>
+      isAsset ? 'assets/keyboard/$category/$name.png' : null;
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) || (other is Pictogram && other.id == id);
+
+  @override
+  int get hashCode => id.hashCode;
+
+  @override
+  String toString() => 'Pictogram($id)';
+}