feat: add multi-account dashboard overview

[wognsdl2]

Summary

• Adds a FinancialAccount model and authenticated /accounts CRUD API.
• Lets expenses attach to an account via account_id with validation.
• Extends /dashboard/summary with account_overview, including monthly income, expenses, net flow, opening balance, and projected balance per account.
• Adds dashboard UI for viewing all accounts in one view and account settings UI for creating accounts.
• Updates OpenAPI/README docs and test coverage.

Verification

• python -m black --check packages/backend/app/models.py packages/backend/app/routes/accounts.py packages/backend/app/routes/__init__.py packages/backend/app/routes/expenses.py packages/backend/app/routes/dashboard.py packages/backend/app/__init__.py packages/backend/tests/conftest.py packages/backend/tests/test_accounts.py packages/backend/tests/test_dashboard.py
• python -m flake8 packages/backend/app/models.py packages/backend/app/routes/accounts.py packages/backend/app/routes/__init__.py packages/backend/app/routes/expenses.py packages/backend/app/routes/dashboard.py packages/backend/app/__init__.py packages/backend/tests/conftest.py packages/backend/tests/test_accounts.py packages/backend/tests/test_dashboard.py
• python -m pytest tests/test_accounts.py tests/test_dashboard.py -q from packages/backend

Note: I could not run the frontend build locally because npm/pnpm are not installed in this environment.

Closes #132