External embed: subtle transparency checker in preview pane

Swapped the embed code generator's live-preview backdrop from
the dev-time bright magenta/yellow checker to the standard
Photoshop-style light-gray/white transparency indicator
(#e5e7eb / #ffffff, 14×14 tiles). Reads as "this is where
transparency shows through" without screaming.

src/index.html

@@ -4678,7 +4678,7 @@
       </div>
 
       <div style="font-weight:600;font-size:.82rem;margin:0 0 4px">Live preview</div>
-      <div id="embedPreviewArea" style="background:repeating-conic-gradient(#ec4899 0% 25%, #fbbf24 0% 50%) 0 0 / 16px 16px;padding:12px;border-radius:6px;margin:0 0 14px;min-height:80px;display:flex;justify-content:center;align-items:center"></div>
+      <div id="embedPreviewArea" style="background:repeating-conic-gradient(#e5e7eb 0% 25%, #ffffff 0% 50%) 0 0 / 14px 14px;padding:12px;border:1px solid var(--line);border-radius:6px;margin:0 0 14px;min-height:80px;display:flex;justify-content:center;align-items:center"></div>
 
       <div style="font-weight:600;font-size:.82rem;margin:0 0 4px">Iframe code</div>
       <textarea id="embedIframeCode" readonly rows="6" style="width:100%;font-family:monospace;font-size:.74rem;box-sizing:border-box;background:#f8fafc;border:1px solid var(--line);border-radius:4px;padding:8px;resize:vertical"></textarea>