- Introduced `getEnvOrPanic` helper to enforce non-empty required environment variables.
- Added strict validation for server (`HOP_SERVER_*`) and client (`HOP_CLIENT_*`) configurations at startup.
- Updated `.env` loader to prioritize OS env vars over `.env` file values.
- Enhanced structured logging for validated environment variables.
- Improved Makefile with `check-env-server` and `check-env-client` targets for build-time validation.
- Defined `hopgate_stream.proto` with message definitions for stream-based DTLS tunneling, including `Request`, `Response`, `StreamOpen`, `StreamData`, `StreamAck`, and `StreamClose`.
- Added `Envelope` container for top-level message encapsulation.
- Integrated Protobuf code generation into the `Makefile` using `protoc` with `protoc-gen-go`.
- Generated Go types under `internal/protocol/pb`.
- Added `github.com/joho/godotenv` for loading `.env` files in server and client.
- Implemented environment variable validation and logging in both main programs.
- Updated Makefile with `.env` export and validation steps for required variables.
- Simplified error handling in `writeErrorPage` rendering logic.
- Implemented custom HTML templates for `400`, `404`, `500`, and `525` error pages with multilingual support.
- Added embedded file system for error page templates and assets.
- Introduced fallback mechanism to serve minimal plain text for missing error templates.
- Integrated TailwindCSS for styling error pages, with a build script in `package.json`.