feat: Implement Data Persistence using Google Sheets API

[rakshitjain23]

Currently, all user-added subjects and attendance data are lost when the user closes or refreshes the application. We need to implement a persistence layer to save and retrieve this data. We will use the Google Sheets API as a free, cloud-based database to store user data in their own Google account.

✅ Acceptance Criteria

• Setup GCP: Create a Google Cloud Platform project, enable the Google Sheets API, and get OAuth 2.0 client credentials.
• Implement Google Auth: Add a Google Sign-In flow that requests the https://www.googleapis.com/auth/spreadsheets scope.
• Sheet Creation: Programmatically create a new Google Sheet named Bunkify Data in a user's Drive on their first login.
• Develop CRUD Functions:
  • Create: When a user adds a new subject, append a new row to the sheet.
  • Read: On app load, fetch all rows to populate the UI.
  • Update: When a user edits a subject or marks attendance, update the corresponding cell(s).
  • Delete: When a user deletes a subject, delete the corresponding row.
• Integrate with Frontend: Replace all local state management with API calls to the new Google Sheets service.

💻 Tech Stack & Guidelines

• Use the official Google API client library for JavaScript (gapi).
• Ensure all data transactions are asynchronous to prevent UI blocking.
• Implement proper error handling for API failures.