Fix Google sign-in failure when Firestore user document is missing

[rahul810050]

Fixed issue: #5

Summary

This PR fixes a Google authentication issue where sign-in fails if the corresponding Firestore user document does not yet exist.

Details

• Firebase Authentication successfully creates the user
• Firestore does not automatically create a /users/{uid} document
• The existing Google sign-in flow attempted to call update() on a non-existent document, causing a runtime error

Changes

• Replaced unsafe Firestore update() calls with set(..., { merge: true }) in the Google sign-in flow
• Ensures user documents are created automatically on first Google login
• Safely updates existing user records on subsequent logins

Impact

• Google sign-in now works reliably for both new and returning users
• No breaking changes
• No behavior changes for email/password authentication