fix(portal): one idempotency key per selected file

A double click or a retry after a network error generated a new key each time
and therefore a duplicate document. The key is now stable until the file
selection changes, so the same upload returns the same verified receipt.

app/portal/dokument/page.tsx

@@ -73,6 +73,10 @@ export default function DocumentsPage() {
   const [uploading, setUploading] = useState(false);
   const [unknownOutcome, setUnknownOutcome] = useState<unknown | null>(null);
   const inputRef = useRef<HTMLInputElement>(null);
+  // One idempotency key per selected file: a retry (double click, network
+  // error, reload-and-pick-the-same-file) returns the SAME document instead of
+  // creating a duplicate. Picking a different file resets it.
+  const uploadKey = useRef<string | null>(null);
 
   const [instruction, setInstruction] = useState('');
   const [fields, setFields] = useState<FieldRow[]>([{ name: 'organization', type: 'string' }]);
@@ -119,7 +123,9 @@ export default function DocumentsPage() {
     const form = new FormData();
     form.set('file', file);
     form.set('project_id', projectId.trim());
-    form.set('idempotency_key', crypto.randomUUID());
+    const key = uploadKey.current ?? crypto.randomUUID();
+    uploadKey.current = key;
+    form.set('idempotency_key', key);
     form.set('classification', 'restricted');
     try {
       const response = await fetch('/api/0pt/documents/upload', {
@@ -293,7 +299,10 @@ export default function DocumentsPage() {
               ref={inputRef}
               type="file"
               className="hidden"
-              onChange={(event) => setFile(event.target.files?.[0] ?? null)}
+              onChange={(event) => {
+                uploadKey.current = null;
+                setFile(event.target.files?.[0] ?? null);
+              }}
             />
           </div>