[FEATURE] Add Bluetooth support to nmrs-gui

[pwsandoval]

Summary

This issue tracks work to add Bluetooth support to nmrs-gui while reusing the existing Wi-Fi UI patterns as much as possible.

The current UX direction includes:

• combined mode by default, showing both Wi-Fi and Bluetooth sections
• --wifi-only / -w for Wi-Fi-only mode
• --bluetooth-only / -b for Bluetooth-only mode

Checklist

• finalize Bluetooth-only mode UX
• finalize combined Wi-Fi + Bluetooth mode UX
• complete Bluetooth device list behavior
• complete Bluetooth details view
• review connect / disconnect / forget actions
• polish interaction and visual consistency

Notes

• The implementation should reuse existing GTK components, styles, and interaction patterns where possible.
• A key backend limitation found during the work: existing Bluetooth support in nmrs was oriented to NetworkManager Bluetooth networking, not generic BlueZ peripherals such as mice, keyboards, or audio devices.
• Backend/API questions for broader Bluetooth support in nmrs are tracked separately.

Related to networkmanager-rs/nmrs#216