Refactor Google Sheets integration

[cpvalente]

This PR refactors the Google Sheets integration in the server application to improve modularity, maintainability, and testability. The original monolithic sheets.service.ts was difficult to extend and test due to its internal state and tight coupling with the @googleapis/sheets library and the device code OAuth flow.

Key changes:

1. GoogleSheetsClient: A new class that wraps the @googleapis/sheets library, providing a cleaner interface for common spreadsheet operations (getSpreadsheet, getValues, batchUpdate).
2. DeviceAuthProvider: A new class that encapsulates the logic and state for the Google OAuth2 device code flow, including polling and cleanup.
3. sheets.service.ts: Refactored to act as a coordinator between the auth provider and the sheets client. It retains all original functionality (initial connection, upload, download, worksheet discovery) but with much cleaner implementation.
4. Testing: Introduced new unit tests for GoogleSheetsClient and DeviceAuthProvider using vitest and mocks. Verified that all existing tests in the project still pass.

These changes aid the future implementation of different authentication methods (e.g., for a cloud service) by providing clear points of extension.

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PR created automatically by Jules for task 6179781105011376301 started by @cpvalente

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