chore: update dependencies and devDependencies in package.json

- Bump react and react-dom to version 19.2.4
- Update @testing-library/jest-dom to version 6.9.1
- Update @testing-library/react to version 16.3.2
- Upgrade @types/node to version 25.1.0
- Upgrade @types/react to version 19.2.10
- Upgrade @types/react-dom to version 19.2.3
- Upgrade @vitest/ui to version 4.0.18
- Upgrade core-js to version 3.48.0
- Upgrade vite-tsconfig-paths to version 6.0.5
- Upgrade vitest to version 4.0.18

refactor: implement MockEventSource class for testing

- Replace vi.fn() mock implementation with a MockEventSource class
- Simplify the mock structure for EventSource in tests

Author: psytraxx

.github/workflows/client.yml

@@ -23,10 +23,10 @@ jobs:
         node-version: [24.x]
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Use Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v5
+        uses: actions/setup-node@v6
         with:
           node-version: ${{ matrix.node-version }}
           cache: "npm"

.github/workflows/gradle.yml

@@ -26,7 +26,7 @@ jobs:
       run:
         working-directory: backend
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up JDK
         uses: actions/setup-java@v5
         with: