Add Xtream DNS failover and customizable multi-view

[antoniovitor10]

Summary

• add automatic Xtream DNS failover for API, EPG, sync, and playback startup
• allow alternative DNS URLs to be managed from the source settings UI
• add an optional environment-based Xtream seed without overwriting UI changes
• add a responsive multi-view page with customizable 1, 2, 3, or 4-screen layouts
• stop hidden players when reducing the layout or leaving multi-view
• keep only one multi-view audio feed active at a time
• document failover, multi-view, connection limits, and seed variables

Validation

• npm test (4 passing tests)
• JavaScript syntax checks
• git diff --check
• desktop and mobile browser validation
• live HLS playback, channel filtering, layout persistence, and stream cleanup verified

Security

• local credentials remain in the ignored .env file
• no provider credentials or private DNS values are included in this pull request