feat: Display detailed submission errors to user

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

webclient/src/Submit.js

@@ -17,13 +17,17 @@ function Submit() {
 		data.append('url', url);
 
 		fetch('/api/submit', { method: 'POST', body: data })
-			.then(res => res.json())
+			.then(res => res.json().then(data => ({ ok: res.ok, data })))
 			.then(
-				(result) => {
-					history.replace('/' + result.nid);
+				({ ok, data }) => {
+					if (ok) {
+						history.replace('/' + data.nid);
+					} else {
+						setProgress(data.error || 'An unknown error occurred.');
+					}
 				},
 				(error) => {
-					setProgress('Error');
+					setProgress(`Error: ${error.toString()}`);
 				}
 			);
 	}