Fix Windows GUI, model preparation and portable EXE builds (#90)

Use a local Qt desktop interface on Windows, verify and prepare model downloads with progress and retry, and build versioned portable executables with release checks. Preserve configured model mirrors and validate application-only refreshes.

Closes #51.

packaging/python-runtime.json

@@ -0,0 +1,5 @@
+{
+  "version": "3.13.15",
+  "url": "https://www.python.org/ftp/python/3.13.15/python-3.13.15-embed-amd64.zip",
+  "sha256": "d1f04d990aee1253d8569e8e5104e30fa9f5fa830899f14843448872d936a2cf"
+}