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/crypto/deriveZkWrapKeyBytes.spec.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from 'vitest';
+import { deriveZkWrapKeyBytes } from '@/utils/crypto/deriveZkWrapKeyBytes';
+import { generateDek } from '@/utils/crypto/generateDek';
+
+describe('deriveZkWrapKeyBytes', () => {
+    const salt = new Uint8Array([8, 7, 6, 5, 4, 3, 2, 1, 0, 9, 8, 7, 6, 5, 4, 3]);
+    const fragmentSecret = generateDek();
+    const password = 'wrap-password';
+
+    it('returns a 32-byte wrap key', async () => {
+        const wrapKey = await deriveZkWrapKeyBytes(password, salt, fragmentSecret);
+
+        expect(wrapKey.byteLength).toBe(32);
+    });
+
+    it('is deterministic for the same inputs', async () => {
+        const first = await deriveZkWrapKeyBytes(password, salt, fragmentSecret);
+        const second = await deriveZkWrapKeyBytes(password, salt, fragmentSecret);
+
+        expect(first).toEqual(second);
+    });
+
+    it('binds the URL fragment secret into the wrap key', async () => {
+        const otherFragment = generateDek();
+
+        const wrapKeyA = await deriveZkWrapKeyBytes(password, salt, fragmentSecret);
+        const wrapKeyB = await deriveZkWrapKeyBytes(password, salt, otherFragment);
+
+        expect(wrapKeyA).not.toEqual(wrapKeyB);
+    });
+});