feat: testing infrastructure (without workflow pending token scope)

- Add vitest, Playwright, ESLint, Prettier configs
- Add unit tests and e2e accessibility/visual tests
- Add MSW mocks and test setup
- Update scripts and .gitignore
- Install required dev dependencies

Note: GitHub Actions workflow will be added after token scope is granted.

.github/workflows/ci.yml

@@ -1,91 +0,0 @@
-name: Admin Frontend CI
-
-on:
-  pull_request:
-    branches: [high-performance]
-  push:
-    branches: [high-performance]
-
-jobs:
-  lint-and-test:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Setup Node.js
-        uses: actions/setup-node@v4
-        with:
-          node-version: "20"
-          cache: "npm"
-
-      - name: Install dependencies
-        run: npm ci
-
-      - name: Run ESLint
-        run: npx eslint . --ext .ts,.tsx
-
-      - name: Check Prettier formatting
-        run: npx prettier --check .
-
-      - name: TypeScript type check
-        run: npx tsc --noEmit
-
-      - name: Run Vitest unit tests
-        run: npm run test:coverage
-
-      - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v4
-        with:
-          files: coverage/lcov.info
-          fail_ci_if_error: false
-
-      - name: Build application
-        run: npm run build
-        env:
-          NODE_ENV: production
-
-  e2e-tests:
-    runs-on: ubuntu-latest
-    needs: lint-and-test
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Setup Node.js
-        uses: actions/setup-node@v4
-        with:
-          node-version: "20"
-          cache: "npm"
-
-      - name: Install dependencies
-        run: npm ci
-
-      - name: Build app for E2E
-        run: npm run build
-        env:
-          NODE_ENV: production
-
-      - name: Start server (preview)
-        run: npm run start:3000 &
-        env:
-          HOST: 0.0.0.0
-          PORT: 3000
-
-      - name: Wait for server
-        run: npx wait-on http://localhost:3000
-
-      - name: Install Playwright browsers
-        run: npx playwright install --with-deps chromium
-
-      - name: Run Playwright accessibility tests
-        run: npm run test:accessibility
-
-      - name: Run Playwright visual regression
-        run: npm run test:visual
-
-      - name: Upload Playwright report
-        if: always()
-        uses: actions/upload-artifact@v4
-        with:
-          name: playwright-report-admin
-          path: playwright-report/
-          retention-days: 14