fix(capture): rotate length-axis rectangle and scale from mask span

Length axis keeps the approved mask, replaces Flip 180 with a ± angle
slider, and sets scale from wood span along the approved direction rather
than bbox.longSidePx. Widths still sample every 6 in perpendicular to that
axis.

frontend/src/capture/MeasurePanel.svelte

@@ -19,7 +19,13 @@
 
 <section class="panel">
   <h1>Measure</h1>
-  <PhotoStage blob={state.source.blob} bbox={state.axis.bbox} heading={state.axis.heading} />
+  <PhotoStage
+    blob={state.source.blob}
+    bbox={state.axis.bbox}
+    maskBitset={state.approvedMask.bitset}
+    maskW={state.source.widthPx > 0 ? state.source.widthPx : 0}
+    maskH={state.source.widthPx > 0 ? state.approvedMask.bitset.length / state.source.widthPx : 0}
+  />
 
   <div class="fields">
     <div class="field">