feat(bank-connector): add connector abstraction, mock implementation, and factory for #266

[macan88]

Problem

There is no abstraction layer for bank integrations, making it impossible to add or swap connectors (import/refresh flows) without touching calling code. Issue #266 requests a connector interface, support for both import and refresh flows, and a mock connector to validate the design.

Solution

Added app/src/lib/bank-connector.ts which exports a BankConnector interface with importTransactions and refreshTransactions methods, a MockBankConnector that satisfies the interface with deterministic in-memory data, and a createConnector factory that decouples consumers from concrete implementations. Real connectors (Plaid, Open Banking, etc.) can be added by extending the ConnectorName union and adding a single case to the factory switch—zero changes required at call sites.

Testing

• Added bank-connector.test.ts with three focused cases:
  • Import flow returns only transactions within the requested date range
  • Refresh flow returns only transactions strictly after the since cursor
  • Factory correctly instantiates a working connector without importing the concrete class

Closes #266