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

.gitignore

@@ -31,6 +31,7 @@ MANIFEST
 #  before PyInstaller builds the exe, so as to inject date/other infos into it.
 *.manifest
 *.spec
+!label_app.spec
 
 # Installer logs
 pip-log.txt
@@ -139,3 +140,8 @@ cython_debug/
 
 # generated label output
 label.png
+
+# build output
+build/
+dist/
+.venv-win/