Migrate from NW.js to pyWebView

[heptau]

The goal is to replace the current NW.js wrapper with pywebview to significantly reduce the application size and simplify the build system.

Tasks:

• Remove NW.js download and packaging logic from Makefile.
• Create a new Python entry point (main.py) using webview to start the web server in a thread and open the window.
• Remove any Node.js specific code (e.g., require) from the frontend JavaScript.
• Verify frontend CSS/JS compatibility with Safari (macOS) and Edge (Windows).
• Update requirements.txt and PyInstaller spec files.

Expected Result:

• Application size reduced from ~250 MB to < 100 MB.
• Simplified Makefile and architecture (pure Python).