Initial scaffold: FocusFlow shared Dart package

Models (Task, Streak, StreakEntry, Reward, User, TimeEstimate, CoworkingRoom, ApiResponse),
enums (EnergyLevel, TaskStatus, RewardType, RewardStyle),
constants (ApiConstants, ErrorCodes, AppLimits), validators, and generated .g.dart files.

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

pubspec.yaml

@@ -0,0 +1,17 @@
+name: focusflow_shared
+description: Shared models, enums, and utilities for FocusFlow ADHD app
+version: 0.1.0
+publish_to: 'none'
+
+environment:
+  sdk: ^3.7.0
+
+dependencies:
+  json_annotation: ^4.9.0
+  equatable: ^2.0.7
+
+dev_dependencies:
+  build_runner: ^2.4.0
+  json_serializable: ^6.8.0
+  lints: ^5.0.0
+  test: ^1.24.0