Add drag-to-reposition for the fixed 3:2 crop

Same fix as latent: the photo area always cover-crops to 3:2 and always
centered, with no way to choose what got cut off. Crop position is now a
normalized 0-1 offset per axis, adjustable by dragging directly on the
canvas whenever the photo's aspect actually leaves slack to pan within.
Pointer movement converts to original-photo pixels via canvas-width/
display-width and outW/cropW (postcard downscales through outW before
compositing, unlike latent's more direct pipeline), so it stays 1:1
regardless of the maxDim downscale or CSS scaling. Resets to centered on
a new photo upload. Verified with a real drag test using far-edge
markers: dragging left/right correctly reveals the opposite edge's
marker in both directions.

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

index.html

@@ -32,6 +32,7 @@
       <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">