fix(frontend): resolve npm install and development server issues

Downgrade eslint to v8.57.0 and TypeScript to v4.9.5 to match
react-scripts@5.0.1 requirements. Force install ajv@8.17.1 and
ajv-keywords@5.1.0 to resolve peer dependency conflicts. Add missing
dependencies (remark-gfm, rehype-highlight, @testing-library/dom).
Update test imports to work with @testing-library/react@16.

frontend/.gitignore

@@ -0,0 +1,22 @@
+# Dependencies
+node_modules
+
+# Build output
+build
+dist
+
+# Environment files
+.env.local
+.env.*.local
+
+# Logs
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# Editor directories
+.vscode
+.idea
+*.swp
+*.swo
+*~