initial commit, I pasted in the GPT sugested code for the project. I read it over and as far as I understand JavaScript it should work properly

content.js

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+if (!document.getElementById("overlay-iframe")) {
+  const iframe = document.createElement("iframe");
+  iframe.src = "https://hiousa.com"; // Change to your overlay site
+  iframe.id = "overlay-iframe";
+  iframe.style.position = "fixed";
+  iframe.style.top = "0";
+  iframe.style.left = "0";
+  iframe.style.width = "100vw";
+  iframe.style.height = "100vh";
+  iframe.style.zIndex = "9999";
+  iframe.style.pointerEvents = "none";
+  iframe.style.opacity = "0.5";
+
+  const slider = document.createElement("input");
+  slider.type = "range";
+  slider.min = 0;
+  slider.max = 100;
+  slider.value = 50;
+  slider.style.position = "fixed";
+  slider.style.top = "10px";
+  slider.style.right = "10px";
+  slider.style.zIndex = "10000";
+
+  slider.oninput = (e) => {
+    iframe.style.opacity = e.target.value / 100;
+  };
+
+  document.body.appendChild(iframe);
+  document.body.appendChild(slider);
+}

manifest.json

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+{
+  "manifest_version": 3,
+  "name": "Live Site Overlay Tool",
+  "version": "1.0",
+  "description": "Overlay one live site over another for pixel-perfect comparison.",
+  "permissions": ["scripting", "activeTab"],
+  "host_permissions": ["<all_urls>"],
+  "action": {
+    "default_title": "Overlay a site"
+  },
+  "content_scripts": [
+    {
+      "matches": ["<all_urls>"],
+      "js": ["content.js"]
+    }
+  ]
+}