refactor(nip-66): use Hammer icon for PoW requirements

Replace Zap (lightning bolt) icon with Hammer icon for proof-of-work
indicators to better represent the mining/work metaphor. Updates both
feed and detail renderers for relay discovery events.

src/components/nostr/kinds/RelayDiscoveryDetailRenderer.tsx

@@ -24,7 +24,7 @@ import {
   Globe,
   Lock,
   CreditCard,
-  Zap,
+  Hammer,
   MapPin,
   Shield,
   Tag,
@@ -207,7 +207,7 @@ export function Kind30166DetailRenderer({ event }: { event: NostrEvent }) {
             {requirements.pow !== undefined && (
               <div className="flex items-center gap-2 text-sm">
                 {requirements.pow ? (
-                  <Zap className="size-4 text-purple-600" />
+                  <Hammer className="size-4 text-purple-600" />
                 ) : (
                   <CheckCircle className="size-4 text-green-600" />
                 )}

src/components/nostr/kinds/RelayDiscoveryRenderer.tsx

@@ -15,7 +15,7 @@ import {
   Globe,
   Lock,
   CreditCard,
-  Zap,
+  Hammer,
 } from "lucide-react";
 import { cn } from "@/lib/utils";
 
@@ -133,7 +133,7 @@ export function Kind30166Renderer({ event }: BaseEventProps) {
               variant="outline"
               className="h-5 px-1.5 gap-1 text-purple-600 border-purple-600/30"
             >
-              <Zap className="size-3" />
+              <Hammer className="size-3" />
               PoW
             </Badge>
           )}