🐛 Fix - preprompt not being passed correctly

The default case should be to use the model preprompt which wasn't being done.

Closes #414

src/routes/conversation/[id]/+server.ts

@@ -102,7 +102,7 @@ export async function POST({ request, fetch, locals, params }) {
 		messages,
 		model,
 		webSearchId: web_search_id,
-		preprompt: settings?.customPrompts?.[model.id],
+		preprompt: settings?.customPrompts?.[model.id] ?? model.preprompt,
 		locals: locals,
 	});