Two critical fixes for Flatpak sandbox environment:
1. PipeWire detection now uses pactl instead of pw-cli
- pw-cli is not available in GNOME Flatpak runtime
- pactl works for both PipeWire and PulseAudio (compatibility layer)
- Fixes missing PipeWire backend in Settings
2. App ID migration now works in Flatpak sandboxes
- Detects Flatpak environment and uses absolute paths
- Accesses old sandbox (com.blitzkriegfc.qbz) via HOME directory
- Added read-only permission to old App ID in manifest
- Properly migrates config/data/cache from old to new sandbox