Fix fm-shim-backend D-Bus name registration and quoting

[assisted-by-ai]

Summary

This PR improves the robustness and security of the fm-shim-backend D-Bus service by fixing shell argument quoting issues in the build script and preventing the service from running in a degraded state when another process owns the D-Bus name.

Key Changes

• Build script quoting: Fixed potential word-splitting issues by storing pkg-config output in arrays and properly expanding them with "${array[@]}" syntax instead of unquoted command substitution
• D-Bus name registration: Added DBUS_NAME_FLAG_DO_NOT_QUEUE flag to prevent the service from being queued when the D-Bus name is unavailable
• Error handling: Changed behavior when another process owns the D-Bus name from a warning with degraded operation to a fatal error, preventing the service from running in an insecure state
• Systemd service: Added NotifyAccess=main to the service unit to properly handle systemd readiness notifications

Implementation Details

The changes ensure that:

1. The build process correctly handles pkg-config flags that may contain spaces or special characters
2. The fm-shim-backend service fails fast if it cannot acquire the required D-Bus name, rather than running with reduced functionality
3. The systemd service properly validates readiness notifications from the main process only

https://claude.ai/code/session_01HJPNe7hjMrPvPtuJmPBhNa