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>

style.css

@@ -144,6 +144,17 @@ p { margin: 0; }
   max-width: 100%;
   max-height: 60vh;
   display: block;
+  touch-action: none;
+}
+
+#canvas.pannable { cursor: grab; }
+#canvas.pannable.panning { cursor: grabbing; }
+
+.pan-hint {
+  text-align: center;
+  font-size: 12px;
+  color: var(--text-dim, #888);
+  margin: -12px 0 20px;
 }
 
 /* ── Controls ───────────────────────────────────────────────────────── */