fix(tests): add Platform.OS cleanup to EditDisplayNameSheet test

Add proper Platform.OS restoration in afterEach hook to prevent test
pollution between tests. Also use Object.defineProperty instead of
direct assignment for Platform.OS mutations, which is the recommended
pattern.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: github-actions[bot]

__tests__/components/sheets/EditDisplayNameSheet.test.tsx

@@ -81,6 +81,9 @@ const mockTheme = {
   glassBorder: 'rgba(255,255,255,0.3)',
 };
 
+// Store original Platform.OS for restoration
+const originalPlatformOS = Platform.OS;
+
 // =============================================================================
 // Tests
 // =============================================================================
@@ -89,6 +92,13 @@ describe('EditDisplayNameSheet', () => {
     jest.clearAllMocks();
   });
 
+  afterEach(() => {
+    // Restore Platform.OS after each test to prevent test pollution
+    Object.defineProperty(Platform, 'OS', {
+      get: () => originalPlatformOS,
+    });
+  });
+
   // ---------------------------------------------------------------------------
   // Rendering Tests
   // ---------------------------------------------------------------------------
@@ -425,7 +435,7 @@ describe('EditDisplayNameSheet', () => {
     });
 
     it('shows confirmation dialog on web when closing with unsaved changes', () => {
-      Platform.OS = 'web';
+      Object.defineProperty(Platform, 'OS', { get: () => 'web' });
       // Mock window.confirm for web platform
       const mockConfirm = jest.fn().mockReturnValue(false);
       global.window = { confirm: mockConfirm } as any;
@@ -458,7 +468,7 @@ describe('EditDisplayNameSheet', () => {
     });
 
     it('shows alert on native when closing with unsaved changes', () => {
-      Platform.OS = 'ios';
+      Object.defineProperty(Platform, 'OS', { get: () => 'ios' });
       const mockAlert = jest.spyOn(Alert, 'alert');
       const mockOnSave = jest.fn();
       const mockOnClose = jest.fn();