feat: add external link confirmation dialog for product URLs

Show an external link icon on ItemCard and CandidateCard that opens a
confirmation dialog before navigating to product URLs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/components/ExternalLinkDialog.tsx

@@ -0,0 +1,65 @@
+import { useEffect } from "react";
+import { useUIStore } from "../stores/uiStore";
+
+export function ExternalLinkDialog() {
+	const externalLinkUrl = useUIStore((s) => s.externalLinkUrl);
+	const closeExternalLink = useUIStore((s) => s.closeExternalLink);
+
+	useEffect(() => {
+		function handleKeyDown(e: KeyboardEvent) {
+			if (e.key === "Escape") closeExternalLink();
+		}
+		if (externalLinkUrl) {
+			document.addEventListener("keydown", handleKeyDown);
+			return () => document.removeEventListener("keydown", handleKeyDown);
+		}
+	}, [externalLinkUrl, closeExternalLink]);
+
+	if (!externalLinkUrl) return null;
+
+	function handleContinue() {
+		if (externalLinkUrl) {
+			window.open(externalLinkUrl, "_blank", "noopener,noreferrer");
+		}
+		closeExternalLink();
+	}
+
+	return (
+		<div className="fixed inset-0 z-50 flex items-center justify-center">
+			<div
+				className="absolute inset-0 bg-black/30"
+				onClick={closeExternalLink}
+				onKeyDown={(e) => {
+					if (e.key === "Escape") closeExternalLink();
+				}}
+			/>
+			<div className="relative bg-white rounded-xl shadow-lg p-6 max-w-sm mx-4 w-full">
+				<h3 className="text-lg font-semibold text-gray-900 mb-2">
+					You are about to leave GearBox
+				</h3>
+				<p className="text-sm text-gray-600 mb-1">
+					You will be redirected to:
+				</p>
+				<p className="text-sm text-blue-600 break-all mb-6">
+					{externalLinkUrl}
+				</p>
+				<div className="flex justify-end gap-3">
+					<button
+						type="button"
+						onClick={closeExternalLink}
+						className="px-4 py-2 text-sm font-medium text-gray-700 bg-gray-100 hover:bg-gray-200 rounded-lg transition-colors"
+					>
+						Cancel
+					</button>
+					<button
+						type="button"
+						onClick={handleContinue}
+						className="px-4 py-2 text-sm font-medium text-white bg-blue-600 hover:bg-blue-700 rounded-lg transition-colors"
+					>
+						Continue
+					</button>
+				</div>
+			</div>
+		</div>
+	);
+}