feat(frontend): show external parent link on remote reply posts

thoughts-frontend/components/thought-card.tsx

@@ -106,6 +106,22 @@ export function ThoughtCard({
             </span>
           </div>
         )}
+        {!thought.replyToId && thought.replyToUrl && (
+          <div className="text-sm text-muted-foreground flex items-center gap-2">
+            <CornerUpLeft className="h-4 w-4 text-primary/70" />
+            <span>
+              Replying to{" "}
+              <a
+                href={thought.replyToUrl}
+                target="_blank"
+                rel="noopener noreferrer"
+                className="hover:underline text-primary text-shadow-sm"
+              >
+                original post ↗
+              </a>
+            </span>
+          </div>
+        )}
       </div>
       <Card className="mt-2">
         <CardHeader className="flex flex-row items-center justify-between space-y-0">