Fix PipeWire detection and App ID migration in Flatpak

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

packaging/flatpak/com.blitzfc.qbz.yml

@@ -25,6 +25,11 @@ finish-args:
   - --filesystem=/media
   - --filesystem=/run/media
 
+  # Migration: Read-only access to old App ID data for one-time migration
+  # This allows copying user data from com.blitzkriegfc.qbz to com.blitzfc.qbz
+  # Can be removed in a future release after users have migrated
+  - --filesystem=~/.var/app/com.blitzkriegfc.qbz:ro
+
   # Required for tray icon to be visible (Tauri writes icon to temp location)
   - --filesystem=xdg-run/tray-icon:create