Add multi-account financial overview

[Iineman2]

Summary

• /claim Multi-account financial overview dashboard #132
• Add a financial account model, account CRUD API, and monthly overview endpoint with net worth, assets, liabilities, currency groups, account-type breakdowns, and recent activity.
• Link expenses to accounts and expose account filtering while preserving unaffiliated expenses.
• Add an authenticated Accounts page with month filtering, account creation, summary cards, balances, and recent activity.
• Document the endpoints in README and OpenAPI, including account-aware expenses.

Validation

• packages/backend: ..venv\Scripts\python -m pytest -q (26 passed, 50 warnings)
• packages/backend: ..venv\Scripts\python -m black --check app tests
• packages/backend: ..venv\Scripts\python -m flake8 app tests
• app: npm test -- Accounts.integration.test.tsx --runInBand (2 passed)
• app: npm run lint
• app: npm run build
• repo: git diff --check