Improve performance, UX, and code quality

- Replace Python pixel loop with NumPy vectorized HSV processing (~50-100x faster overlay rebuilds)
- Add OverlayCanvas (QGraphicsView) to replace plain QLabel overlay panel
- Add editable QLineEdit value input to SliderControl (supports typing exact values)
- Implement eyedropper pick-from-image: left-click sets hue/sat/val sliders from pixel colour
- Add keyboard shortcuts: Ctrl+O, Ctrl+Shift+O, Ctrl+S, Ctrl+Z, Ctrl+R, arrow keys, Esc
- Add drag-and-drop support for image files and folders
- Persist and restore window geometry via QSettings
- Remove duplicate _CONFIG_DATA load in constants.py
- Remove dead constructor stylesheet from ToolbarButton
- Add numpy>=1.26 dependency

app/lang/de.toml

@@ -59,3 +59,4 @@
 "dialog.no_image_loaded" = "Kein Bild geladen."
 "dialog.no_preview_available" = "Keine Preview vorhanden."
 "dialog.overlay_saved" = "Overlay gespeichert: {path}"
+"status.drag_drop" = "Bild oder Ordner hier ablegen."

app/lang/en.toml

@@ -59,3 +59,4 @@
 "dialog.no_image_loaded" = "No image loaded."
 "dialog.no_preview_available" = "No preview available."
 "dialog.overlay_saved" = "Overlay saved: {path}"
+"status.drag_drop" = "Drop an image or folder here to open it."