Enable parallel UI test execution via per-session data isolation

Each test class now gets a unique session ID (UUID) passed to the app
via UI_TEST_SESSION_ID environment variable. The app uses this to:

- Route GroupUserDefaults to a session-specific UserDefaults suite,
  preventing tests from clobbering each other's AppStorage state
- Create an in-memory SwiftData container instead of the shared
  on-disk App Group store, eliminating SQLite contention

Refactored 8 test classes that bypassed BaseUITestCase.setUp() with
custom launch args — they now use overridable `localeArguments` and
`extraLaunchArguments` properties, keeping session ID injection
centralized. Added `relaunchApp(resetState:bypassSubscription:)` to
BaseUITestCase for tests that need mid-test relaunch with different
subscription state.

Includes a ParallelUITests.xctestplan with class-level parallelism
enabled and random execution ordering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Tests iOS/Helpers/BaseUITestCase.swift

@@ -3,7 +3,8 @@
 //  Tests iOS
 //
 //  Base class for all UI tests. Handles launch arguments,
-//  state reset, and screenshot capture on failure.
+//  state reset, screenshot capture on failure, and parallel
+//  test isolation via per-session data sandboxing.
 //
 
 import XCTest
@@ -12,6 +13,13 @@ class BaseUITestCase: XCTestCase {
 
     var app: XCUIApplication!
 
+    // MARK: - Parallel Test Isolation
+
+    /// Unique session ID for this test class instance.
+    /// Passed to the app via environment so each parallel runner gets
+    /// its own UserDefaults suite and in-memory SwiftData container.
+    private(set) var testSessionID: String = UUID().uuidString
+
     // MARK: - Configuration (override in subclasses)
 
     /// Fixture to seed. Override to use a specific data set.
@@ -26,6 +34,15 @@ class BaseUITestCase: XCTestCase {
     /// Whether to force the trial to be expired. Default: false.
     var expireTrial: Bool { false }
 
+    /// Override to change the test locale/language.
+    /// Default: English (US). Locale tests override this instead of setUp().
+    var localeArguments: [String] { ["-AppleLanguages", "(en)", "-AppleLocale", "en_US"] }
+
+    /// Extra launch arguments for tests needing special settings
+    /// (accessibility sizes, reduce motion, high contrast, etc.).
+    /// Override in subclasses instead of overriding setUp().
+    var extraLaunchArguments: [String] { [] }
+
     // MARK: - Lifecycle
 
     override func setUp() {
@@ -46,7 +63,8 @@ class BaseUITestCase: XCTestCase {
     // MARK: - Launch Configuration
 
     private func buildLaunchArguments(resetState: Bool) -> [String] {
-        var args = ["--ui-testing", "--disable-animations", "-AppleLanguages", "(en)", "-AppleLocale", "en_US"]
+        var args = ["--ui-testing", "--disable-animations"]
+        args.append(contentsOf: localeArguments)
         if resetState {
             args.append("--reset-state")
         }
@@ -59,11 +77,13 @@ class BaseUITestCase: XCTestCase {
         if expireTrial {
             args.append("--expire-trial")
         }
+        args.append(contentsOf: extraLaunchArguments)
         return args
     }
 
     private func buildLaunchEnvironment() -> [String: String] {
         var env = [String: String]()
+        env["UI_TEST_SESSION_ID"] = testSessionID
         if let fixture = seedFixture {
             env["UI_TEST_FIXTURE"] = fixture
         }
@@ -90,6 +110,26 @@ class BaseUITestCase: XCTestCase {
         return application
     }
 
+    /// Relaunch the app with custom bypass setting, preserving the session ID.
+    /// Use when a test needs to toggle subscription bypass mid-test.
+    @discardableResult
+    func relaunchApp(resetState: Bool, bypassSubscription overrideBypass: Bool) -> XCUIApplication {
+        app.terminate()
+        let application = XCUIApplication()
+        var args = ["--ui-testing", "--disable-animations"]
+        args.append(contentsOf: localeArguments)
+        if resetState { args.append("--reset-state") }
+        if overrideBypass { args.append("--bypass-subscription") }
+        if skipOnboarding { args.append("--skip-onboarding") }
+        if expireTrial { args.append("--expire-trial") }
+        args.append(contentsOf: extraLaunchArguments)
+        application.launchArguments = args
+        application.launchEnvironment = buildLaunchEnvironment()
+        application.launch()
+        app = application
+        return application
+    }
+
     @discardableResult
     func relaunchPreservingState() -> XCUIApplication {
         app.terminate()