[Bug] Imported private key is cleared when returning from the agent harness setup screen

After importing a private key and continuing to the **Set up your agent harnesses** screen, clicking **Back** returns to the private-key screen with an empty input. The previously entered key is no longer populated.

**To Reproduce:**

1. Open the Buzz onboarding flow.
2. Click **Use an existing key**.
3. Enter a valid private key.
4. Continue to **Set up your agent harnesses**.
5. Click **Back**.
6. Notice that the private-key input is empty.

**Expected behavior:**
The previously entered private key should remain populated and masked when the user returns to the private-key screen.

**Environment (please complete the following information):**

* OS: Windows

Start by reproducing the Buzz onboarding flow through the private-key and agent-harness setup screens, then trace how the key input state changes when Back is clicked. Done means the previously entered key remains populated and masked after returning to the private-key screen; the issue does not name a file or test to run.

Written by the indexing model from the issue text.