Windows build scripts + updated PyInstaller spec

build_windows.bat + build_windows.ps1: one-command build on Win10/11.
Creates venv, installs deps + PyInstaller, rebuilds STIG catalog cache,
produces dist/netguard.exe. ~85 MB single file (includes Python +
Flask + netmiko + pywebview + the 1,005-rule DISA catalog).

Spec updates:
- console=False (no CMD window on GUI launch; CLI still inherits parent
  console stdout)
- hiddenimports for netmiko/paramiko/cryptography/openpyxl/webview
  ecosystem (all reachable only via runtime imports)
- bundle stigs/ tree + catalog cache into datas so binary is self-
  contained out of the box
- exclude pytest/IPython/tkinter to trim size

docs/BUILD_WINDOWS.md: prereqs, one-command build, test commands,
SmartScreen + AppLocker considerations, EV cert signing path, size
breakdown, troubleshooting.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
