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>

example/focusflow_shared_example.dart

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+import 'package:focusflow_shared/focusflow_shared.dart';
+
+void main() {
+  // Create a task
+  final task = Task(
+    id: 'task-001',
+    userId: 'user-001',
+    title: 'Write project proposal',
+    status: TaskStatus.pending.apiValue,
+    energyLevel: EnergyLevel.medium.name,
+    priority: 75,
+    estimatedMinutes: 30,
+    tags: ['work', 'writing'],
+    createdAt: DateTime.now(),
+  );
+
+  print('Task: ${task.title} (priority: ${task.priority})');
+  print('Status: ${TaskStatus.pending.displayName}');
+  print('Energy: ${EnergyLevel.medium.displayName}');
+
+  // Validate
+  final titleError = TaskValidator.validateTitle(task.title);
+  print('Title valid: ${titleError == null}');
+
+  // API path
+  print('Task endpoint: ${ApiConstants.task(task.id)}');
+
+  // Limits
+  print('Free tier max tasks: ${AppLimits.freeMaxActiveTasks}');
+}