feat(observability): guard navigator.clipboard and handle rejection in copyId

Adds availability guard (navigator.clipboard may be undefined in non-HTTPS
contexts) and a rejection handler so clipboard-denied errors are silently
caught rather than becoming unhandled promise rejections. Tests cover the
success feedback and the silent-failure path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+error.svelte

@@ -7,10 +7,16 @@ let copied = $state(false);
 function copyId() {
 	const id = page.error?.errorId;
 	if (!id) return;
-	navigator.clipboard.writeText(id).then(() => {
-		copied = true;
-		setTimeout(() => (copied = false), 2000);
-	});
+	if (!navigator.clipboard) return;
+	navigator.clipboard.writeText(id).then(
+		() => {
+			copied = true;
+			setTimeout(() => (copied = false), 2000);
+		},
+		() => {
+			/* clipboard denied or unavailable — select-all on the <code> element remains */
+		}
+	);
 }
 </script>