Close all 25 codex audit findings across KMP, iOS, and Android

Remediate all P0-S priority findings from cross-platform architecture audit:
- Harden token storage with EncryptedSharedPreferences (Android) and Keychain (iOS)
- Add SSL pinning and certificate validation to API clients
- Fix subscription cache race conditions and add thread-safe access
- Add input validation for document uploads and file type restrictions
- Refactor DocumentApi to use proper multipart upload flow
- Add rate limiting awareness and retry logic to API layer
- Harden subscription tier enforcement in SubscriptionHelper
- Add biometric prompt for sensitive actions (Login, Onboarding)
- Fix notification permission handling and device registration
- Add UI test infrastructure (page objects, fixtures, smoke tests)
- Add CI workflow for mobile builds

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

iosApp/CaseraUITests/PageObjects/RegisterScreen.swift

@@ -0,0 +1,86 @@
+import XCTest
+
+/// Page object for the registration screen.
+///
+/// Uses accessibility identifiers from `AccessibilityIdentifiers.Authentication`
+/// to locate registration form elements and perform sign-up actions.
+class RegisterScreen: BaseScreen {
+
+    // MARK: - Elements
+
+    var usernameField: XCUIElement {
+        app.textFields[AccessibilityIdentifiers.Authentication.registerUsernameField]
+    }
+
+    var emailField: XCUIElement {
+        app.textFields[AccessibilityIdentifiers.Authentication.registerEmailField]
+    }
+
+    var passwordField: XCUIElement {
+        app.secureTextFields[AccessibilityIdentifiers.Authentication.registerPasswordField]
+    }
+
+    var confirmPasswordField: XCUIElement {
+        app.secureTextFields[AccessibilityIdentifiers.Authentication.registerConfirmPasswordField]
+    }
+
+    var registerButton: XCUIElement {
+        app.buttons[AccessibilityIdentifiers.Authentication.registerButton]
+    }
+
+    var cancelButton: XCUIElement {
+        app.buttons[AccessibilityIdentifiers.Authentication.registerCancelButton]
+    }
+
+    /// Fallback element lookup for the register/create account button using predicate
+    var registerButtonByLabel: XCUIElement {
+        app.buttons.containing(NSPredicate(format: "label CONTAINS[c] 'Register' OR label CONTAINS[c] 'Create Account'")).firstMatch
+    }
+
+    override var isDisplayed: Bool {
+        // Registration screen is visible if any of the register-specific fields exist
+        let usernameExists = usernameField.waitForExistence(timeout: timeout)
+        let emailExists = emailField.exists
+        return usernameExists || emailExists
+    }
+
+    // MARK: - Actions
+
+    /// Fills in the registration form and submits it.
+    /// Returns a MainTabScreen assuming successful registration leads to the main app.
+    @discardableResult
+    func register(username: String, email: String, password: String) -> MainTabScreen {
+        waitForElement(usernameField).tap()
+        usernameField.typeText(username)
+
+        emailField.tap()
+        emailField.typeText(email)
+
+        passwordField.tap()
+        passwordField.typeText(password)
+
+        confirmPasswordField.tap()
+        confirmPasswordField.typeText(password)
+
+        // Try accessibility identifier first, fall back to label search
+        if registerButton.exists {
+            registerButton.tap()
+        } else {
+            registerButtonByLabel.tap()
+        }
+
+        return MainTabScreen(app: app)
+    }
+
+    /// Taps cancel to return to the login screen.
+    @discardableResult
+    func tapCancel() -> LoginScreen {
+        if cancelButton.exists {
+            cancelButton.tap()
+        } else {
+            // Fall back to navigation back button
+            tapBackButton()
+        }
+        return LoginScreen(app: app)
+    }
+}