feat: Multi-account Dashboard and Data Export (CSV/JSON)

[ViVaLaDaniel]

Summary

This PR implements two major feature requests: Multi-account management (#132) and Data Export capabilities (#589).

Changes

🏦 Multi-account Management (#132)

• Backend: Added FinancialAccount model with AccountType (Bank, Cash, Credit Card, Investment).
• Backend API: Implemented full CRUD routes for financial accounts.
• Backend Data: Updated Expense, Bill, and RecurringExpense to optionally reference a specific account.
• Frontend: Created useAccounts hook and API client.
• Dashboard: Integrated an "Accounts" overview section that shows real-time balances for all user accounts.

📥 Data Export (#589)

• Frontend: Added "Export" section to the Expenses page.
• Format Support: Implemented client-side generation and download for both CSV and JSON formats.
• Context: Exports are based on currently loaded expense data, respecting active filters.