Rewrite label designer as an item-based canvas editor

- Items (text blocks, emoji) are placed freely: click to select, drag to
  move, arrow-key nudge, rotate to any angle, resize, duplicate, fit/center
- Emoji picker now covers the full emoji set via emoji-data-python
  (~1,900, 9 categories, name search, hover shows the name); thumbnails
  are rendered by Pillow with Noto Color Emoji so ZWJ sequences and flags
  display exactly as they print
- Designs save/open as JSON; last design restored on launch

pyproject.toml

@@ -9,6 +9,7 @@ python = "~3.11"
 pyserial = "^3.5"
 pillow = "^10.1.0"
 click = "^8.1.7"
+emoji-data-python = "^1.6"
 
 [build-system]
 requires = ["poetry-core"]