Fix failing unit tests

* This PR caused the Sidebar.test.ts file tests to fail because TransformStream is not found.
* TransformStream exists in the Node version I'm testing with (20), but it still isn't found by Jest
* Turns out it is a problem with Jest, and the workaround is the simple package jest-fixed-jsdom, which subclasses JSDOMEnvironment testing environment, placing this and several other dependencies in its global object.

* In package.json
  - add jest-fixed-jsdom as a devDependency

* In jest.config.cjs
  - change testEnvironment to jest-fixed-jsdom

client/jest.config.cjs

@@ -1,6 +1,6 @@
 module.exports = {
   preset: "ts-jest",
-  testEnvironment: "jsdom",
+  testEnvironment: "jest-fixed-jsdom",
   moduleNameMapper: {
     "^@/(.*)$": "<rootDir>/src/$1",
     "\\.css$": "<rootDir>/src/__mocks__/styleMock.js",