fix: handle declined and error states in grant interaction page

The grant interaction page (payment-confirmation.tsx) previously
showed the success UI with a green checkmark for all states,
including declined grants and server errors. Users saw "Payment
Complete!" even when their grant was rejected.

Changes:
- Check the `result` query parameter for interaction outcome
  (grant_rejected, etc.) and display appropriate error UI
- Use the loader's `success` field to conditionally render
  success or error state with distinct icons and messaging
- Rename the component from PaymentComplete to GrantInteraction
  to better reflect its purpose
- Display the specific error message from the loader

Closes #480

Author: Onyx2406

frontend/app/routes/payment-confirmation.tsx

@@ -11,7 +11,7 @@ import { validatePaymentParams } from '~/utils/validate.server'
 export const meta: MetaFunction = () => {
   return [
     { title: 'Grant Interaction' },
-    { name: 'description', content: 'Interaction success' },
+    { name: 'description', content: 'Grant interaction result' },
   ]
 }
 
@@ -20,6 +20,19 @@ export async function loader({ request, context }: LoaderFunctionArgs) {
   const url = new URL(request.url)
   const params = Object.fromEntries([...url.searchParams])
 
+  // Check for interaction result parameters (e.g., result=grant_rejected)
+  const interactionResult = params.result
+  if (interactionResult && interactionResult !== 'grant_approved') {
+    return data({
+      success: false,
+      error:
+        interactionResult === 'grant_rejected'
+          ? 'The grant request was declined.'
+          : `Interaction failed: ${interactionResult}`,
+      params,
+    })
+  }
+
   const validation = validatePaymentParams(params)
   if (!validation.success) {
     return data({
@@ -58,19 +71,64 @@ export async function loader({ request, context }: LoaderFunctionArgs) {
   }
 }
 
-export default function PaymentComplete() {
-  const { params } = useLoaderData<typeof loader>()
+export default function GrantInteraction() {
+  const loaderData = useLoaderData<typeof loader>()
   const hasPostedMessage = useRef(false)
+  const isSuccess = loaderData.success
 
   useEffect(() => {
     if (hasPostedMessage.current) return
 
     hasPostedMessage.current = true
     if (window.opener) {
-      window.opener.postMessage({ type: 'GRANT_INTERACTION', ...params }, '*')
+      window.opener.postMessage(
+        { type: 'GRANT_INTERACTION', ...loaderData.params },
+        '*',
+      )
     }
   }, [])
 
+  if (!isSuccess) {
+    return (
+      <div className="min-h-screen flex items-center justify-center bg-gray-50">
+        <div className="max-w-md w-full space-y-8 p-8">
+          <div className="text-center">
+            <div className="mx-auto flex items-center justify-center h-24 w-24 rounded-full bg-red-100 mb-6">
+              <svg
+                className="h-12 w-12 text-red-600"
+                fill="none"
+                stroke="currentColor"
+                viewBox="0 0 24 24"
+                xmlns="http://www.w3.org/2000/svg"
+              >
+                <path
+                  strokeLinecap="round"
+                  strokeLinejoin="round"
+                  strokeWidth={2}
+                  d="M6 18L18 6M6 6l12 12"
+                />
+              </svg>
+            </div>
+
+            <h2 className="text-3xl font-bold text-gray-900 mb-4">
+              Interaction Failed
+            </h2>
+
+            <p className="text-gray-600 mb-8">
+              {'error' in loaderData && loaderData.error
+                ? loaderData.error
+                : 'Something went wrong. Please try again.'}
+            </p>
+
+            <p className="text-gray-400 text-sm">
+              You can close this window.
+            </p>
+          </div>
+        </div>
+      </div>
+    )
+  }
+
   return (
     <div className="min-h-screen flex items-center justify-center bg-gray-50">
       <div className="max-w-md w-full space-y-8 p-8">