feat(web): favicon + toast feedback (polish)

Add a vinyl-record favicon (icon.svg) — fixes the favicon.ico 404 console
error. Lightweight toast system (ToastHost + toast()) fired on request, wanted
add, search, follow, and want actions so submits give explicit feedback.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/layout.tsx

@@ -3,6 +3,7 @@ import "./design.css";
 import { fraunces } from "./fonts";
 import { Masthead } from "./_ui/masthead";
 import { ContentsNav } from "./_ui/contents-nav";
+import { ToastHost } from "./_ui/toast";
 
 export const metadata = { title: "Lyra" };
 
@@ -15,6 +16,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
           <ContentsNav />
           {children}
         </div>
+        <ToastHost />
       </body>
     </html>
   );