Allow HTTP for local Navidrome servers

[lostf1sh]

Summary

• Permit plain HTTP connections for local Navidrome/Subsonic servers while keeping remote hosts on HTTPS.
• Update the login screen copy and default prefill to reflect local-network HTTP setups.
• Enable cleartext traffic at the app level so local server connections can succeed.
• Add unit coverage for accepted local HTTP URLs and rejected public HTTP URLs.

Testing

• Added/updated unit tests for Navidrome URL validation.
• Not run (not requested).

Might fixes #27