fix: Fix policy document format to send as object instead of double-stringified JSON

- Change generatePolicyFromTemplate to return object instead of string
- Update CreatePolicyRequest interface to accept string | object for policy
- Update CreatePolicyDialog to stringify policy object only when needed
- Fix QuickStartWizard to send policy object directly to API

This fixes the 'Proxy error' when creating policies as the backend was
receiving double-stringified JSON which failed validation.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/components/Policies/CreatePolicyDialog.tsx

@@ -74,7 +74,7 @@ const CreatePolicyDialog: React.FC<CreatePolicyDialogProps> = ({
   const generatePreview = () => {
     if (bucketName && selectedTemplate) {
       const generatedPolicy = policyService.generatePolicyFromTemplate(selectedTemplate, bucketName);
-      setCustomPolicy(generatedPolicy);
+      setCustomPolicy(JSON.stringify(generatedPolicy, null, 2));
     }
   };
 
@@ -111,7 +111,7 @@ const CreatePolicyDialog: React.FC<CreatePolicyDialogProps> = ({
         setError('Bucket name is required');
         return;
       }
-      policyToCreate = policyService.generatePolicyFromTemplate(selectedTemplate, bucketName);
+      policyToCreate = JSON.stringify(policyService.generatePolicyFromTemplate(selectedTemplate, bucketName), null, 2);
     }
 
     if (!validatePolicy()) {