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/routers/__init__.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter
-from app.routers import tasks, categories, tags, user_settings, habits, anniversaries, auth
+from app.routers import tasks, categories, tags, user_settings, habits, anniversaries, auth, goals
 
 api_router = APIRouter()
 
@@ -10,3 +10,4 @@ api_router.include_router(tags.router)
 api_router.include_router(user_settings.router)
 api_router.include_router(habits.router)
 api_router.include_router(anniversaries.router)
+api_router.include_router(goals.router)