Implements the mocked-up sidebar refinements: each section header now
carries a small marker matching its on-canvas handle (cyan square for
crop, green rotate knob, orange diamond for padding), teaching the
editor's color language without explanatory text, and a Preview section
at the bottom of the sidebar renders the actual output composite - fill
background, rotated source clipped to the crop, padded canvas, no
overlays - live while dragging any handle.
Browser-verified: all three markers render, sections appear in order
(Crop, Rotate, Padding & mask, View & reset, Preview), and the preview
canvas resizes and repaints when rotation and padding change, with fill
color in the padding corners and image content in the center.
ausboss
c4d609cf7e