Fix failing auth module tests by introducing AuthValidator

[Copilot]

The test suite had a placeholder testExample() with no assertions and no AuthValidator type to test against, leaving auth input validation untested.

Changes

• ThriveApp/AuthValidator.swift (new): Pure Foundation struct with two static validators:
  • isValidEmail(_:) — regex-based RFC-style email check via NSPredicate
  • isValidPassword(_:) — enforces Firebase's 6-character minimum

AuthValidator.isValidEmail("user@example.com")  // true
AuthValidator.isValidEmail("userexample.com")   // false
AuthValidator.isValidPassword("abc123")         // true
AuthValidator.isValidPassword("12345")          // false

• ThriveAppTests/ThriveAppTests.swift: Replaced empty testExample() with 9 XCTest cases covering well-formed inputs, boundary values, and invalid inputs for both validators.

• ThriveApp.xcodeproj/project.pbxproj: Registered AuthValidator.swift in the ThriveApp target so it's reachable via @testable import ThriveApp.