Introduce RectangleDrawing and wire rectangle drawing through the app: add RectangleDrawing model with normalized rect helpers; implement start/update/finish flows in DrawingProvider; add erasing support for rectangle strokes via _isPointNearRectangleStroke; render rectangles (solid and dashed) in DrawingPainter for both existing elements and the current element. Update InteractivePainter to use a single pen state watch and branch on PenMode to start/update/finish rectangles vs free draw. Update Hive adapter metadata to remove the obsolete isRectangle field and adjust serialization indices. Minor cleanups: remove an unused import/field in drawing_painter and mark DeleteArea as const where applicable.