Potential fix for code scanning alert no. 17: Uncontrolled data used in path expression

[maxirmx]

Potential fix for https://github.com/sw-consulting/media-pi.device/security/code-scanning/17

To fix this safely without changing core functionality (still allowing configurable subdirectories), validate that playlistDestination resolves under a fixed safe base directory before saving it. The best place is in HandleConfigurationUpdate (input validation boundary), and optionally enforce again at use sites for defense-in-depth.
For the shown code, update internal/agent/menu.go where cleanDestination is computed:

• Define a constant safe base dir (for example /var/media-pi).
• Resolve both safe base and candidate destination to absolute, cleaned paths.
• Reject if destination is not equal to base and not under base + path separator.
• Keep storing the validated absolute path (cleanDestination) in config as before.

This prevents traversal and arbitrary absolute path selection while preserving intended behavior of writing playlists into configurable folders under the application directory.

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