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

build-windows.ps1

@@ -0,0 +1,9 @@
+# Build NiimbotLabel.exe on Windows.
+#   Requires Python 3.10+ from python.org (with tcl/tk, which the installer includes by default).
+#   Run from this folder in PowerShell:  .\build-windows.ps1
+$ErrorActionPreference = "Stop"
+if (-not (Test-Path .venv-win)) { py -3 -m venv .venv-win }
+.\.venv-win\Scripts\python -m pip install --upgrade pip
+.\.venv-win\Scripts\pip install -r requirements.txt pyinstaller
+.\.venv-win\Scripts\pyinstaller --noconfirm --clean label_app.spec
+Write-Host "`nBuilt: dist\NiimbotLabel.exe"