feat: hide chain spec cards on mobile, keep mainnet details visible

Mobile users only see the Mainnet detail card (WebSocket endpoint,
Chain ID, explorer button, services grid). The individual chain
cards (Staging, Testnet, Beta, etc.) and subdomains box are hidden
on mobile and visible on md+ screens.

web/src/components/ChainSpecs.tsx

@@ -134,7 +134,8 @@ const ChainSpecs: React.FC = () => {
           </p>
         </div>
 
-        <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-4 gap-4 mb-8">
+        {/* Chain cards grid - hidden on mobile, visible on md+ */}
+        <div className="hidden md:grid md:grid-cols-2 lg:grid-cols-4 gap-4 mb-8">
           {chainSpecs.map((spec) => (
             <div
               key={spec.id}
@@ -157,9 +158,9 @@ const ChainSpecs: React.FC = () => {
                   {spec.type}
                 </span>
               </div>
-              
+
               <h3 className="text-white font-semibold mb-2">{spec.name}</h3>
-              
+
               <div className="flex items-center text-sm text-gray-400">
                 <Server className="w-3 h-3 mr-1" />
                 <span>{spec.validators} validators</span>