Introduce environment variables to globally and individually control background jobs:
- `WARPBOX_JOBS_ENABLED` to toggle all background workers.
- `WARPBOX_CLEANUP_ENABLED` to toggle the expired box cleanup job.
- `WARPBOX_THUMBNAIL_ENABLED` to toggle the thumbnail generation job.
Refactor background tasks into a dedicated `backend/libs/jobs` package, allowing jobs to be registered, scheduled, and conditionally run based on the new configuration flags. Additionally, update the default maximum upload size in `.env.example` to 16GB and document the new settings in the README.
Initialize the repository with the core Go backend architecture and a frontend mockup for warpbox.dev, a self-hosted file-sharing application.
- Set up Go backend modules for configuration, HTTP server, middleware, handlers, and templates.
- Add local development scripts, environment templates, and basic project configuration.
- Include a React-based frontend mockup under the docs directory.
