Refactor niche type declaration in form components

- Updated the niche property in BatchForm, ExtensiveForm, and GenerationForm components to use 'as const' for improved type safety and consistency in TypeScript.

frontend/components/generation/BatchForm.tsx

@@ -28,7 +28,7 @@ export const BatchForm: React.FC<BatchFormProps> = ({
   } = useForm({
     resolver: zodResolver(generateBatchSchema),
     defaultValues: {
-      niche: "home_insurance" as Niche,
+      niche: "home_insurance" as const,
       count: 5,
       images_per_ad: 1,
       image_model: null,

frontend/components/generation/ExtensiveForm.tsx

@@ -57,7 +57,7 @@ export const ExtensiveForm: React.FC<ExtensiveFormProps> = ({
   } = useForm<ExtensiveFormData>({
     resolver: zodResolver(extensiveSchema),
     defaultValues: {
-      niche: "home_insurance" as Niche,
+      niche: "home_insurance" as const,
       custom_niche: "",
       target_audience: "",
       offer: "",

frontend/components/generation/GenerationForm.tsx

@@ -28,7 +28,7 @@ export const GenerationForm: React.FC<GenerationFormProps> = ({
   } = useForm({
     resolver: zodResolver(generateAdSchema),
     defaultValues: {
-      niche: "home_insurance" as Niche,
+      niche: "home_insurance" as const,
       num_images: 1,
       image_model: null,
       target_audience: "",