feat(chat): Add remote user public key fetching for cross-node messaging

- Implement remote user detection based on @domain format in recipient handle
- Add HTTP/HTTPS protocol selection logic for localhost vs production domains
- Fetch remote user public keys from their node's /api/users endpoint
- Maintain local user key lookup for non-remote recipients
- Add error handling and debug logging for remote key fetch failures
- Enable end-to-end encryption support for federated chat across nodes

src/app/api/swarm/chat/messages/route.ts

@@ -71,11 +71,30 @@ export async function GET(request: NextRequest) {
     
     console.log('[Messages API] Fetching recipient key for:', recipientHandle);
     
-    // Fetch recipient's chat public key
+    // Check if this is a remote user (has @domain)
+    const isRemote = recipientHandle.includes('@');
+    
+    if (isRemote) {
+      // Remote user - fetch from their node
+      const [handle, domain] = recipientHandle.split('@');
+      try {
+        const protocol = domain.includes('localhost') ? 'http' : 'https';
+        const response = await fetch(`${protocol}://${domain}/api/users/${handle}`);
+        if (response.ok) {
+          const data = await response.json();
+          recipientPublicKey = data.user?.chatPublicKey || null;
+          console.log('[Messages API] Fetched remote recipient key:', !!recipientPublicKey);
+        }
+      } catch (error) {
+        console.error('[Messages API] Failed to fetch remote recipient key:', error);
+      }
+    } else {
+      // Local user
       const recipientUser = await db.query.users.findFirst({
         where: eq(users.handle, recipientHandle),
       });
       recipientPublicKey = recipientUser?.chatPublicKey || null;
+    }
     
     console.log('[Messages API] Recipient public key found:', !!recipientPublicKey);