fix: smaller header logo (sm size) + remove stale .ico favicon fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -4,7 +4,6 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
-    <link rel="icon" type="image/x-icon" href="/favicon.ico" />
     <title>Cohorta</title>
     <meta name="description" content="AI-powered synthetic focus groups for product research" />
     <meta name="author" content="AImpress" />

src/components/layout/Header.tsx

@@ -65,7 +65,7 @@ export default function Header() {
         >
           {/* Logo */}
           <Link to="/" className="flex-shrink-0">
-            <Logo withWordmark />
+            <Logo withWordmark size="sm" />
           </Link>
 
           {/* Pill nav — desktop */}