Add drag-to-reposition for aggressive crops

Non-original aspects (square, 4:5, 16:9) always centered the crop with
no way to choose what got cut off. Crop position is now a normalized
0-1 offset per axis instead of a fixed /2 center, and dragging directly
on the canvas -- only when the current aspect actually leaves slack to
pan within -- adjusts it. Pointer movement converts to rotated-image
pixels via the crop-width-to-display-width ratio, so it stays 1:1
regardless of the maxDim downscale or the canvas's own CSS scaling.
Resets to centered on aspect or rotation change, since old offsets
don't make sense against new dimensions. Verified with a real drag test:
a marker cropped out at the centered default becomes visible after
dragging, and the "original" aspect (no crop, no slack) correctly
leaves panning disabled.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

index.html

@@ -26,6 +26,7 @@
       <div class="canvas-wrap">
         <canvas id="canvas"></canvas>
       </div>
+      <p class="pan-hint" id="pan-hint" hidden>Drag the photo to reposition the crop</p>
 
       <div class="controls">
         <div class="control-group">