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>

lib/src/enums/energy_level.dart

@@ -0,0 +1,13 @@
+enum EnergyLevel {
+  low,
+  medium,
+  high;
+
+  String get displayName {
+    switch (this) {
+      case EnergyLevel.low: return 'Low Energy';
+      case EnergyLevel.medium: return 'Medium Energy';
+      case EnergyLevel.high: return 'High Energy';
+    }
+  }
+}