Function promptForCredentials

promptForCredentials(): Promise<CredentialsInput>
Prompt user for credentials (handle and password).

Uses two-step input process:
1. Prompt for handle with validation
2. Prompt for password with validation
Either step can be cancelled, which cancels the entire flow.
Returns Promise<CredentialsInput>
Credentials object, or undefined if cancelled
Example
```
const creds = await promptForCredentials();
if (creds) {
  await login(creds.handle, creds.password);
} else {
  console.log('Login cancelled');
}
```
- Defined in src/ui/dialogs/credentialsInput.ts:118