fix: prevent metadata corruption for weba and flac files

[thumb2086]

• Skip metadata writing for flac files to prevent file corruption
• FLAC files have strict header requirements where writing metadata after download can corrupt the sync code and make files unplayable
• Update download_manager_provider.dart to check for both weba and flac extensions
• Update server/routes/playback.dart to check for both weba and flac extensions
• Add explanatory comments documenting why metadata writing is skipped for these formats
• Refactor extension checking to use variable assignment for clarity