Auth: Username Prompt for Apple & Google

<img width="509" height="727" alt="Image" src="https://github.com/user-attachments/assets/0f0f79d5-115a-48d4-9807-7e18c92c2ba9" />

As you can see, we have all currently got the same name and username on our profile after creating an account with cherry...

Figma Design for ideal account creation screen Pending.- Basic screen with UserName / FullName and hardcoded profile picture

We currently don't have any option for users to input unique usernames so their account/ profile may be identified. Implementing this in MVP future-proofs profile features and avoids downstream complexities, whilst keeping consistency across users.

Why

As a new user signing in with Google or Apple
I want to be asked to choose a unique username
And be easily identified by our full name So that I have a consistent identity in the app and can be identified across features

Acceptance Criteria

• Given I sign in using Google or Apple for the first time
  When my Firebase user record is created
  Then I should be shown a screen prompting me to choose a username

• Given I enter a username and tap confirm
  When the username is valid and available
  Then it should be saved to Firestore and associated with my UID

• Given I try to enter a taken or invalid username
  When I submit the form
  Then I should see a validation message and not be allowed to proceed

• Given I return to the app on a later login
  When I sign in again via Google/Apple
  Then I should bypass the username screen if my username has already been saved

Proposed Implementation

• After successful Google/Apple sign-in, check if a username field exists for the user in Firestore
  • If not, redirect to a username entry screen
• Username must be:
  • 3–20 characters
  • Alphanumeric (with optional underscores)
  • Unique across all users (Firestore query check)
• Store under /users/{uid}/username
• Prevent access to other app sections until a username is provided
• Can reuse existing UI styling from other input screens

Backend

• Ensure username is stored in Firestore at /users/{uid}/username
• Implement a method to check that the username is globally unique:
  • Option 1: Cloud Function that queries all users and returns availability
  • Option 2: Use a dedicated /usernames/{username} collection for atomic writes and lookups
• If using an API layer (Swagger), define endpoint for submitting and validating username
• Handle concurrent write edge cases to prevent duplicates