[Bug] Third-party miners always show post-setup login warning even when default credentials are sufficient

### Describe the bug

When adding third-party miners, the UI always shows `You will need to log in after setup`, even when the miner is using default credentials that Proto Fleet can already handle automatically.

This makes the setup flow look like manual authentication will always be required, even when no additional login should be needed.

### Preconditions

- At least one third-party miner is discoverable on the network
- A simple way to reproduce is to remove one from the current fleet list first

### Steps to reproduce

1. Go to `Miners`
2. Click `Add miners`
3. Click `Find miners`

### Expected behavior

- The message should not assume the user will need to log in after setup
- Proto Fleet should either:
- suppress the warning when default credentials are sufficient, or
- check whether valid credentials are already available or stored

### Proto Fleet version

`nightly-20260703-a7b00e867a12-13-g04a0c8c5-dirty`

### Environment

- OS: macOS
- Browser: Chrome

### Screenshots & screen recordings

https://github.com/user-attachments/assets/96e8b41c-9821-4698-8249-b0850c64616e

Start in the Miners flow by reproducing the issue through Add miners and Find miners with a discoverable third-party miner. Trace where the post-setup login warning is rendered and how default or stored credentials are determined; done means the warning is not shown when Proto Fleet can authenticate automatically, while still appearing when manual login is required.

Written by the indexing model from the issue text.