fix: handle 5xx server errors with retry message in playback network

[LargeModGames]

This pull request primarily improves error handling for Spotify server errors in playback networking, and makes a minor import order fix in the test module. The most significant change is the addition of user-friendly messaging and retry logic for temporary Spotify server issues.

Playback network error handling improvements:

• Added detection for Spotify server 5xx errors (502, 503, 504, "Gateway Timeout", "Service Unavailable", "Bad Gateway") in Network's playback error handling. When such an error is detected, a clear status message is shown to the user, and polling is automatically retried after a short delay, improving user experience during transient outages.

Test module cleanup:

• Fixed the import order in the test module of src/core/app.rs by moving the test_helpers import to follow the module declaration, improving code organization.

[Copilot]

Pull request overview

This PR improves resiliency of playback polling by detecting Spotify server-side 5xx failures during me/player fetches and showing a user-facing “retrying” status message instead of surfacing the error immediately. It also performs a small test-module import reordering for cleanliness.

Changes:

• Add special-case handling for Spotify 5xx (502/503/504 + reason phrases) during playback polling to show a retry status and avoid error escalation.
• Reset playback polling timer on 5xx errors to trigger an automatic retry on the next poll interval.
• Reorder imports in src/core/app.rs test module.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/infra/network/playback.rs	Adds 5xx detection and user-friendly retry messaging in current playback polling error path.
src/core/app.rs	Adjusts import ordering inside the #[cfg(test)] module.