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/Persisence/SharedModelContainer.swift

@@ -36,6 +36,15 @@ enum SharedModelContainer {
     /// - Returns: Configured ModelContainer
     /// - Throws: SharedModelContainerError if creation fails
     static func create(useCloudKit: Bool = true) throws -> ModelContainer {
+        // When UI testing, use in-memory storage for parallel test isolation.
+        // Each test process gets its own empty container — no shared on-disk state.
+        // Check ProcessInfo directly to avoid depending on UITestMode (not in widget targets).
+        if ProcessInfo.processInfo.arguments.contains("--ui-testing") {
+            let schema = Schema([MoodEntryModel.self])
+            let config = ModelConfiguration(schema: schema, isStoredInMemoryOnly: true, cloudKitDatabase: .none)
+            return try ModelContainer(for: schema, configurations: [config])
+        }
+
         let schema = Schema([MoodEntryModel.self])
         let storeURL = try Self.storeURL