fix(fe): axis preview reactive now depends on approvedMask

The axis-preview draw reactive only depended on the image being complete, so
it fired before approvedMask was populated (canvas stayed at the default
300x150 and nothing drew). Add cap.approvedMask to the reactive deps so the
sheet-removed photo + tape line render once the frozen mask is present.

frontend/src/capture/SessionCapture.svelte

@@ -79,7 +79,7 @@
     axisObjUrl = URL.createObjectURL(cap.source.blob);
   }
 
-  $: if (kind === 'axis' && axisImg?.complete && axisImg.naturalWidth > 0) {
+  $: if (kind === 'axis' && cap.approvedMask && axisImg?.complete && axisImg.naturalWidth > 0) {
     drawAxisPreview();
   }