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>

style.css

@@ -128,6 +128,17 @@ 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);
+  margin: -12px 0 20px;
 }
 
 /* ── Controls ───────────────────────────────────────────────────────── */