255: Parametrize backend URL and adjust workflow files

[SebastianNowak01]

This pull request includes changes to support the use of environment variables for backend URL configuration in the frontend application. The changes involve updating workflow files, environment example files, and frontend source code.

Environment variable support:

• .github/workflows/frontend.yml: Added VITE_BACKEND_URL environment variable to the workflow configuration.
• .github/workflows/static.yml: Added VITE_BACKEND_URL environment variable to the static workflow configuration.
• frontend/.env.example: Added VITE_BACKEND_URL to the example environment variables file.

Frontend source code updates:

• frontend/src/constants/backendUrl.ts: Created a new file to export the BACKEND_URL constant from environment variables.
• frontend/src/providers/WebsocketProvider.tsx: Updated the WebSocket connection URL to use the BACKEND_URL constant instead of a hardcoded localhost URL.

[deepsource-io]

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