Rename public/icon.png to logo.png to fix blurry icon

Next.js App Router serves app/icon.png (32x32 favicon) at /icon.png,
shadowing the 256x256 public/icon.png. Rename to /logo.png so the
high-res image is served for the site logo.

web/app/components/site-header.tsx

@@ -28,7 +28,7 @@ export function SiteHeader({
               <>
                 <Link href="/" className="flex items-center gap-2.5">
                   <img
-                    src="/icon.png"
+                    src="/logo.png"
                     alt="cmux"
                     width={24}
                     height={24}

web/app/page.tsx

@@ -12,7 +12,7 @@ export default function Home() {
         {/* Header */}
         <div className="flex items-center gap-4 mb-10" data-dev="header">
           <img
-            src="/icon.png"
+            src="/logo.png"
             alt="cmux icon"
             width={48}
             height={48}