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>

Shared/UITestMode.swift

@@ -42,6 +42,12 @@ enum UITestMode {
         ProcessInfo.processInfo.arguments.contains("--expire-trial")
     }
 
+    /// Unique session ID for parallel test isolation.
+    /// Each test class gets its own session, ensuring no shared state between parallel test runners.
+    static var sessionID: String? {
+        ProcessInfo.processInfo.environment["UI_TEST_SESSION_ID"]
+    }
+
     /// Seed fixture name if provided (via environment variable)
     static var seedFixture: String? {
         ProcessInfo.processInfo.environment["UI_TEST_FIXTURE"]
@@ -93,8 +99,9 @@ enum UITestMode {
     @MainActor
     private static func resetAppState() {
         let defaults = GroupUserDefaults.groupDefaults
-        // Clear group user defaults using the suite domain name
-        defaults.removePersistentDomain(forName: Constants.currentGroupShareId)
+        // Clear group user defaults using the session-specific or shared suite domain name
+        let suiteName = GroupUserDefaults.currentSuiteName
+        defaults.removePersistentDomain(forName: suiteName)
 
         // Explicitly clear subscription cache keys that may survive removePersistentDomain
         // on app group suites (known reliability issue).