Stop login retry loop after successful authentication

[ameel]

Successful authentication is repeated when RETRY_ATTEMPTS is greater than zero because the login loop has no success exit. With the default retry count of one, Zotify creates a second Spotify session immediately after the first session succeeds.

This adds a break after login() returns successfully. Connection failures still follow the existing retry behavior.

Validation:

• regression test confirms one login call after success;
• regression test confirms a failed login is still retried;
• performed a complete track download using the saved Spotify credentials after applying the fix.