feat: add PyQt6 GUI with progress tracking and cleaner logs

- Added gui.py containing a modern PyQt6 interface.
- Updated server_pack_builder.py to support GUI integration via --gui flag.
- Implemented aggregate download size tracking and granular mod processing progress.
- Simplified CLI logging to reduce noise (silenced urllib3, shortened messages).
- Updated README.md with GUI usage instructions.
- Added PyQt6 to requirements.txt.

Author: devsker

README.md

@@ -28,6 +28,13 @@ Download and filter a Modrinth modpack directly from a URL or slug:
 python server_pack_builder.py --modrinth-url https://modrinth.com/modpack/fabric-boosted
 ```
 
+### GUI Mode
+Launch the graphical user interface for an easy-to-use experience:
+
+```bash
+python server_pack_builder.py --gui
+```
+
 ### Arguments
 
 #### Common