refactor: improve test structure for Bun compatibility

- Update all test files to use named mock variables instead of inline mocks
- Clear mock history in beforeEach instead of using mock.restore()
- Add default mock implementations stored at module level
- Document that tests should not use --randomize flag due to mock.module() limitations
- All 89 unit tests pass consistently without randomization

package.json

@@ -7,10 +7,14 @@
     "build": "next build",
     "start": "next start",
     "lint": "bun run eslint",
-    "test": "bun test",
-    "test:run": "bun test",
-    "test:acceptance": "bun x playwright test src/__tests__/e2e/",
-    "test:acceptance:headed": "bun x playwright test src/__tests__/e2e/ --headed",
+    "test": "bun test 'src/__tests__/unit/**' 'src/__tests__/*.test.tsx' 'src/__tests__/auth-simple.test.ts'",
+    "test:unit": "bun test src/__tests__/unit/",
+    "test:component": "bun test src/__tests__/*.test.tsx",
+    "test:run": "bun test 'src/__tests__/unit/**' 'src/__tests__/*.test.tsx' 'src/__tests__/auth-simple.test.ts'",
+    "test:randomize": "bun test src/__tests__/unit/ --randomize",
+    "test:unit:sequential": "bun test src/__tests__/unit/ --max-concurrency=1",
+    "test:acceptance": "bun x playwright test e2e/",
+    "test:acceptance:headed": "bun x playwright test e2e/ --headed",
     "db:switch": "bun run scripts/switch-database.js",
     "db:setup-postgres": "bun run scripts/setup-postgres.js",
     "db:setup-dev": "bun run scripts/setup-postgres.js",