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/LongTranslationTests.swift

@@ -10,25 +10,7 @@ import XCTest
 final class LongTranslationTests: BaseUITestCase {
     override var seedFixture: String? { "single_mood" }
     override var bypassSubscription: Bool { true }
-
-    override func setUp() {
-        // Do NOT call super — we need German locale (known for long compound words)
-        continueAfterFailure = false
-
-        let application = XCUIApplication()
-        let args: [String] = [
-            "--ui-testing", "--disable-animations",
-            "--reset-state",
-            "--bypass-subscription",
-            "--skip-onboarding",
-            "-AppleLanguages", "(de)",
-            "-AppleLocale", "de_DE"
-        ]
-        application.launchArguments = args
-        application.launchEnvironment = ["UI_TEST_FIXTURE": "single_mood"]
-        application.launch()
-        app = application
-    }
+    override var localeArguments: [String] { ["-AppleLanguages", "(de)", "-AppleLocale", "de_DE"] }
 
     /// TC-138: German locale with long compound words renders without crashes.
     /// Navigates through all tabs to ensure no layout truncation causes issues.