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

label_app.spec

@@ -0,0 +1,48 @@
+# -*- mode: python ; coding: utf-8 -*-
+# PyInstaller spec for the Niimbot label designer.
+#   Windows:  pyinstaller label_app.spec   -> dist/NiimbotLabel.exe
+#   Linux:    pyinstaller label_app.spec   -> dist/NiimbotLabel
+# Build on the target OS; PyInstaller does not cross-compile.
+
+import sys
+from pathlib import Path
+
+from PyInstaller.utils.hooks import collect_data_files
+
+ROOT = Path(SPECPATH)
+
+datas = [
+    (str(ROOT / "assets" / "fonts"), "assets/fonts"),
+    *collect_data_files("emoji_data_python"),  # emoji.json
+    *collect_data_files("ttkbootstrap"),  # themes
+]
+
+# Pillow's Windows wheel needs fribidi.dll for raqm text shaping (emoji sequences)
+binaries = []
+if sys.platform == "win32":
+    binaries.append((str(ROOT / "assets" / "win" / "fribidi.dll"), "."))
+
+a = Analysis(
+    [str(ROOT / "label_app.py")],
+    pathex=[str(ROOT)],
+    datas=datas,
+    binaries=binaries,
+    hiddenimports=["serial.tools.list_ports", "PIL.ImageTk", "PIL._tkinter_finder"],
+    excludes=["numpy", "matplotlib", "scipy", "pandas", "IPython"],
+    noarchive=False,
+)
+pyz = PYZ(a.pure)
+
+exe = EXE(
+    pyz,
+    a.scripts,
+    a.binaries,
+    a.datas,
+    [],
+    name="NiimbotLabel",
+    icon=str(ROOT / "assets" / "icon.ico") if sys.platform == "win32" else None,
+    console=False,  # GUI app: no console window
+    upx=False,
+    strip=False,
+    onefile=True,
+)