Add Tkinter label designer with emoji picker

label_app.py: label size presets, multi-line text with auto-fit / bold /
alignment, tabbed emoji picker (rendered via Noto Color Emoji,
gamma-darkened and dithered for the thermal head), live 1-bit preview,
save PNG, print in a background thread. Settings persist in
~/.config/niimprint-label/.

main.py is the earlier minimal GUI it supersedes.

pyproject.toml

@@ -23,3 +23,4 @@ select = ["I", "F", "E"]
 
 [tool.ruff.lint.per-file-ignores]
 "__init__.py" = ["F401"]
+"label_app.py" = ["E501"]  # emoji tables