add optional Argon2id password wrap for zk uploads.

Wrap the image DEK for password-protected batches, keep the URL fragment as a separate secret, and extend prepare/decrypt helpers with tests.

frontend/src/utils/upload/prepareZkUploadRequest.spec.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest';
 import { base64ToBytes } from '@/utils/encoding/base64ToBytes';
 import { buildZkViewerPath } from '@/utils/share/buildZkViewerPath';
+import { decryptPasswordProtectedZkUpload } from '@/utils/upload/decryptPasswordProtectedZkUpload';
 import { decryptZkUpload } from '@/utils/upload/decryptZkUpload';
 import { prepareZkUploadRequest } from '@/utils/upload/prepareZkUploadRequest';
 
@@ -58,4 +59,54 @@ describe('prepareZkUploadRequest', () => {
 
         expect(recovered).toEqual(plaintext);
     });
+
+    describe('password protected', () => {
+        const passwordOptions = {
+            expiresAt: null,
+            maxViews: null,
+            passwordProtected: true as const,
+            password: 'batch-secret'
+        };
+
+        it('includes wrap fields and omits password from the POST body', async () => {
+            const { request, fragmentKey } = await prepareZkUploadRequest(new Uint8Array([1]), passwordOptions);
+
+            expect(request.passwordProtected).toBe(true);
+            expect(request.wrappedDek).toMatch(/^[A-Za-z0-9+/]+=*$/);
+            expect(request.wrapIv).toMatch(/^[A-Za-z0-9+/]+=*$/);
+            expect(request.salt).toMatch(/^[A-Za-z0-9+/]+=*$/);
+            expect(request).not.toHaveProperty('password');
+            expect(fragmentKey).toMatch(/^[A-Za-z0-9_-]+$/);
+        });
+
+        it('does not decrypt with URL fragment alone', async () => {
+            const plaintext = new TextEncoder().encode('protected');
+            const { request, fragmentKey } = await prepareZkUploadRequest(plaintext, passwordOptions);
+
+            await expect(
+                decryptZkUpload(
+                    base64ToBytes(request.ciphertext),
+                    base64ToBytes(request.iv),
+                    fragmentKey
+                )
+            ).rejects.toThrow();
+        });
+
+        it('roundtrips with fragment key and password', async () => {
+            const plaintext = new TextEncoder().encode('protected-payload');
+            const { request, fragmentKey } = await prepareZkUploadRequest(plaintext, passwordOptions);
+
+            const recovered = await decryptPasswordProtectedZkUpload(
+                base64ToBytes(request.ciphertext),
+                base64ToBytes(request.iv),
+                fragmentKey,
+                passwordOptions.password,
+                base64ToBytes(request.wrappedDek!),
+                base64ToBytes(request.wrapIv!),
+                base64ToBytes(request.salt!)
+            );
+
+            expect(recovered).toEqual(plaintext);
+        });
+    });
 });