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/Random.swift

@@ -25,7 +25,26 @@ struct Constants {
 }
 
 struct GroupUserDefaults {
+    /// Whether the current process is a UI test session with an isolation ID.
+    /// Inlined from ProcessInfo to avoid depending on UITestMode (which isn't in widget targets).
+    private static var uiTestSessionID: String? {
+        guard ProcessInfo.processInfo.arguments.contains("--ui-testing") else { return nil }
+        return ProcessInfo.processInfo.environment["UI_TEST_SESSION_ID"]
+    }
+
+    /// The suite name currently in use. Used by resetAppState() to clear the correct domain.
+    static var currentSuiteName: String {
+        if let sessionID = uiTestSessionID {
+            return "uitest.\(sessionID)"
+        }
+        return Constants.currentGroupShareId
+    }
+
     static var groupDefaults: UserDefaults {
+        // When UI testing with a session ID, use a per-session suite for parallel isolation.
+        if let sessionID = uiTestSessionID {
+            return UserDefaults(suiteName: "uitest.\(sessionID)") ?? .standard
+        }
 #if DEBUG
         return UserDefaults(suiteName: Constants.groupShareIdDebug) ?? .standard
 #else