fix(capture): show bg-removed cutout on Measure Zoom/Pan immediately

Tape-mode PhotoStage skipped mask checkerboard painting, so Zoom/Pan
depended on async createMaskedPreviewBlob — a hang/fail left the stage
without a cutout. Paint mask overlay in tape mode, mount PhotoStage on
source+bitset right away (swap to alpha PNG when ready), checkerboard
stage backdrop, and prefer HTML canvas for the preview encode.

frontend/src/capture/MeasurePanel.svelte

@@ -228,19 +228,21 @@
       {/if}
     </p>
 
-    {#if maskedPreview}
-      <PhotoStage
-        mode="tape"
-        blob={maskedPreview}
-        {tape}
-        {cropFill}
-        {gestureMode}
-        onTapeChange={onTapeChange}
-      />
-    {:else}
-      <div class="preview-loading" role="status">
-        {previewError || 'Preparing background-removed preview…'}
-      </div>
+    <!-- Cutout first: source + maskBitset paints checkerboard immediately (tape mode).
+         When createMaskedPreviewBlob finishes, swap to alpha PNG (same footprint). -->
+    <PhotoStage
+      mode="tape"
+      blob={maskedPreview ?? state.source.blob}
+      maskBitset={maskedPreview ? null : state.approvedMask.bitset}
+      {maskW}
+      {maskH}
+      {tape}
+      {cropFill}
+      {gestureMode}
+      onTapeChange={onTapeChange}
+    />
+    {#if previewError}
+      <p class="preview-err" role="status">{previewError}</p>
     {/if}
 
     <div class="controls">
@@ -449,7 +451,7 @@
   .mode-btn.active { background: #00e5ff; color: #002; }
   .mode-hint { margin: 0 0 10px; }
   .nudge { color: #e8a; font-size: 0.74rem; margin: 6px 0 0; line-height: 1.45; }
-  .preview-loading { min-height: 420px; border: 1px solid #333; border-radius: 8px; display: flex; align-items: center; justify-content: center; color: #999; text-align: center; padding: 16px; box-sizing: border-box; }
+  .preview-err { color: #ffb4a2; font-size: 0.85rem; margin: 8px 0 0; }
   .controls { margin-top: 16px; display: flex; flex-direction: column; gap: 14px; }
   .ctl-label { font-size: 0.78rem; color: #aaa; margin: 0 0 6px; }
   .ctl-label .sub { font-size: 0.7rem; color: #777; }