Update default preview image location

Remove duplicate image, move to previews folder, and update
existing paths.

unfurler/src/routes.ts

@@ -83,7 +83,7 @@ const routes = {
 
 const networks = {
   bitcoin: {
-    fallbackImg: '/resources/previews/dashboard.png',
+    fallbackImg: '/resources/previews/mempool-space-preview.png',
     routes: {
       ...routes // all routes supported
     }
@@ -147,4 +147,4 @@ export function matchRoute(network: string, path: string): Match {
   }
 
   return match;
-}
+}