feat(connectors): pluggable BankConnector interface, registry, and mock connector for Indian bank sync

[macan88]

Problem

The codebase has no abstraction for bank integrations — every potential connector would need to be hardcoded into the sync logic with no shared contract, making it impossible to add AA/API-provider connections without rewriting core sync code each time.

Solution

Adds three files that together form a minimal, extensible connector layer:

File	Role
app/src/lib/connectors/base.ts	BankConnector interface + Transaction/ImportResult types + typed error classes
app/src/lib/connectors/registry.ts	ConnectorRegistry singleton — register(), getConnector(), listConnectors()
app/src/lib/connectors/mock.ts	MockBankConnector — deterministic INR transaction data, cursor pagination, forced-failure auth path

Key design decisions:

• Cursor-based pagination on importTransactions(cursor?) prevents duplicate transactions on retry and supports incremental sync without refetching full history.
• refreshSync() separate from importTransactions() so AA connectors can re-validate consent on background polls without affecting user-triggered imports.
• Registry throws on duplicate ID rather than silently overwriting — avoids hard-to-debug production data integrity issues.
• In-memory credentials only — real connector implementations must proxy auth through the backend; no API keys in localStorage (documents the security requirement explicitly in base.ts).
• _resetForTesting() on registry keeps unit tests isolated without module re-imports.

A real AA-provider connector (e.g., Finvu / Setu AA) can be added by implementing BankConnector and calling ConnectorRegistry.register('aa-finvu', FinvuConnector) — zero changes to existing files required.

Testing

• Inline test suite documented in mock.ts (runnable via vitest) covering:
  • Registry returns MockBankConnector instance for id "mock"
  • Duplicate registration throws
  • Unregistered ID throws with helpful message
  • importTransactions() returns transactions with id, amount, date, currency: 'INR'
  • Cursor pagination produces non-overlapping pages; hasMore correct at end
  • authenticate({ fail_auth: 'true' }) throws ConnectorAuthError

Closes #75