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/task.py

@@ -21,3 +21,4 @@ class Task(Base):
     # 关联关系
     category = relationship("Category", back_populates="tasks")
     tags = relationship("Tag", secondary="task_tags", back_populates="tasks")
+    goals = relationship("Goal", secondary="goal_tasks", back_populates="tasks")