feat: add goal management module (long-term goals with phases, milestones, reviews)

Backend:
- Goal model: title, description, status (active/paused/completed/abandoned),
  progress (auto-computed from milestones), target_date, category, color, icon
- GoalStep model: unified phase/milestone with parent nesting
- GoalReview model: periodic reflection with rating
- goal_tasks M2M: link existing tasks to goals
- /api/goals CRUD + steps CRUD + reviews + task linking + status toggle
- Progress auto-calculated from milestone completion ratio

Frontend:
- GoalPage: card grid with progress bars, status filter
- GoalDetailPage: step tree (phases > milestones), reviews, linked tasks
- GoalDialog: create/edit form with color/icon picker
- Goal navigation in AppHeader
- useGoalStore: full Pinia store for all goal operations

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/models/__init__.py

@@ -4,9 +4,11 @@ from app.models.tag import Tag, task_tags
 from app.models.user_settings import UserSettings
 from app.models.habit import HabitGroup, Habit, HabitCheckin
 from app.models.anniversary import AnniversaryCategory, Anniversary
+from app.models.goal import Goal, GoalStep, GoalReview, goal_tasks
 
 __all__ = [
     "Task", "Category", "Tag", "task_tags", "UserSettings",
     "HabitGroup", "Habit", "HabitCheckin",
     "AnniversaryCategory", "Anniversary",
+    "Goal", "GoalStep", "GoalReview", "goal_tasks",
 ]