Add experimental Bluetooth transport backend for KDE Connect

[FarisZR]

Summary

• add a new bluetooth backend (src/service/backends/bluetooth.js) that integrates with BlueZ ProfileManager1 and Device1
• register the KDE Connect RFCOMM profile UUID and reuse the bundled SDP record for service registration
• add Bluetooth identity/certificate exchange and certificate trust validation for paired devices
• register backend in manager and support bluetooth:// reconnect/identify flow
• extend connect dialog to allow manual Bluetooth address connection
• persist Bluetooth certificate trust and improve Bluetooth encryption info display
• document implementation and current limitations in docs/bluetooth-transport.md

Notes

• this implementation is marked experimental
• payload transfer channels (upload/download) are not implemented yet for Bluetooth in this first iteration

Validation

• npx eslint src/service/backends/bluetooth.js src/service/manager.js src/service/device.js src/preferences/service.js (passes with JSDoc warnings only)