feat: add savings goals tracking with milestones

[hikali123456789]

Goal-based savings tracking & milestones (#133)

Implements the savings goals feature with full CRUD support and milestone tracking.

Changes

Backend:

• Added goals and goal_milestones database tables with indexes
• Added Goal and GoalMilestone SQLAlchemy models with relationship
• Added full REST API for goals (/goals):
  • GET /goals - List all goals for user (with milestones)
  • POST /goals - Create a goal (with optional milestones)
  • PATCH /goals/ - Update a goal
  • DELETE /goals/ - Soft delete (set active=False)
  • POST /goals//contribute - Add amount to current_amount (auto-checks milestones)
  • GET /goals//milestones - List milestones
  • POST /goals//milestones - Add a milestone
  • DELETE /goals//milestones/ - Delete a milestone
• Added backend tests

Frontend:

• Added Goals page with progress bars, milestone tracking
• Added goal creation/edit/contribute dialogs
• Added Goals navigation link in Navbar
• Added Goals route in App.tsx

Acceptance Criteria Met

• Production ready implementation
• Includes tests
• Follows existing codebase patterns

/claim #133