auth: sign-up with phone or Google + optional reference code/password

Goal

Implement sign-up flow with either phone number or Google, including profile fields and optional password.

Requirements

• Support Phone Number sign-up.
• Support Google sign-up.
• Collect and store:
  • Name (required)
  • Reference code (optional)
  • Password (optional)
• If user does not add password, allow later login without password by checking phone existence in database.

Firebase Scope

• Use Firebase Authentication providers:
  • Phone
  • Google
• Persist app user profile in Firestore (users collection).

Data Model (minimum)

users/{uid}

• uid
• name
• phoneNumber (nullable for Google-first users)
• referenceCode (nullable)
• authProviders (e.g., ["phone"], ["google"], or both)
• passwordEnabled (bool)
• passwordHash (nullable, never store raw password)
• createdAt
• updatedAt

Functional Behavior

• Phone sign-up path creates/updates user profile.
• Google sign-up path creates/updates user profile.
• If password entered during sign-up:
  • store secure hash
  • set passwordEnabled = true
• If password omitted:
  • set passwordEnabled = false

Acceptance Criteria

• User can complete sign-up with phone or Google.
• Name and optional reference code are saved correctly.
• Optional password behavior is implemented exactly as requested.
• Firestore user document is created/updated reliably.
• Validation and error states are handled for duplicate/invalid input.

Notes

• Password must be hashed before storage.
• Security rules should restrict user document access by authenticated owner.

Start by locating the Flutter authentication entry points and the Firebase Authentication and Firestore integration; this issue names no files or tests. Trace the Phone and Google flows against the users/{uid} data model, then verify completion using the stated validation, password-hashing, security-rule, and acceptance criteria.