feat: Goal-based savings tracking

[chengyixu]

Summary

Implements goal-based savings tracking feature allowing users to create, manage, and visually track progress toward financial saving targets. Closes #133.

/claim #133

Backend (Flask/SQLAlchemy/PostgreSQL)

• Models: SavingsGoal (name, description, target/current amount, currency, target date, icon, color, status) and SavingsContribution (amount, note, date) with full lifecycle management (ACTIVE → COMPLETED → CANCELLED)
• API Endpoints (/savings):
  • GET /savings — List goals with status filter (ACTIVE/COMPLETED/ALL)
  • GET /savings/<id> — Single goal with contribution history
  • POST /savings — Create goal (auto-defaults to user's preferred currency)
  • PATCH /savings/<id> — Update goal details
  • DELETE /savings/<id> — Delete goal and contributions
  • POST /savings/<id>/contributions — Add deposit (auto-completes at 100%)
  • POST /savings/<id>/withdraw — Withdraw with balance validation
  • GET /savings/summary — Aggregate stats across all goals
• Schema: PostgreSQL migration with indexed savings_goals and savings_contributions tables
• Tests: 9 comprehensive tests covering CRUD, contributions, auto-complete, withdrawals, validation, status filtering, and currency defaults

Frontend (React/TypeScript/Tailwind)

• API Client (api/savings.ts): Type-safe client for all endpoints
• SavingsGoals Page (pages/SavingsGoals.tsx):
  • Summary cards: total saved, total target, overall progress bar, completed count
  • Goal cards with color-coded progress bars, deposit/withdraw/edit/delete actions
  • Target date tracking with days remaining + monthly savings needed calculation
  • Status filter tabs (Active / Completed / All)
  • Create/edit/contribute/withdraw dialogs with validation
  • Empty state with onboarding CTA
• Navigation: "Savings" link added to navbar between Budgets and Bills
• Routing: /savings route registered as protected route
• Test: Frontend integration test suite

Documentation

• OpenAPI spec updated with all 8 savings endpoints and 6 new schemas

Test plan

• Backend: pytest tests/test_savings.py — 9 tests pass (CRUD, contributions, auto-complete, withdraw, validation, filtering, currency)
• Frontend: jest SavingsGoals.integration.test — renders goals, empty state, progress bars, filtering
• Manual: Create goal → add contributions → verify progress bar updates → complete goal → verify status change
• Manual: Test withdraw with insufficient balance → verify error
• Manual: Navigate to /savings from navbar → verify protected route

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