fix: preserve prompt text after generation

Remove setPrompt('') and setStructuredPrompt({}) from handleSubmit
so users can iterate on prompts without retyping.

frontend/src/components/Generator.tsx

@@ -96,8 +96,6 @@ export default function Generator({ onGenerate }: Props) {
         throw new Error(data.error)
       }
 
-      setPrompt('')
-      setStructuredPrompt({})
       onGenerate()
     } catch (err) {
       setError(err instanceof Error ? err.message : 'Something went wrong')