Add PyInstaller packaging for Windows and Linux

- label_app.spec: one-file build bundling fonts, emoji data, ttkbootstrap
  themes and (on Windows) fribidi.dll so Pillow can shape emoji sequences
- build-windows.ps1 helper; assets/icon.ico app icon
- Fonts resolved from assets/fonts first, then system dirs, so the app
  runs from a bundle or a checkout on any OS
- Bump Pillow pin to 12.3

pyproject.toml

@@ -7,7 +7,7 @@ authors = []
 [tool.poetry.dependencies]
 python = "~3.11"
 pyserial = "^3.5"
-pillow = "^10.1.0"
+pillow = ">=10.1"
 click = "^8.1.7"
 emoji-data-python = "^1.6"
 ttkbootstrap = "^1.10"