Add custom device names via Rename Device menu item

[ATLMCW]

Summary

• Adds a Rename Device… menu item that lets users assign a custom label (e.g. "My iPhone", "My Apple Watch") to the monitored BLE device
• Custom name replaces the raw auto-detected name in the device scan list, while still showing MAC address and RSSI
• Custom name is prefixed to the RSSI readout at the top of the menu (e.g. My iPhone: −65dBm)
• Names are persisted in UserDefaults keyed by CoreBluetooth UUID and survive restarts
• Menu item is greyed out until a device is selected; leaving the name blank clears it

Test plan

• Select a device from the Device submenu
• Click "Rename Device…" and enter a custom name — confirm it appears in the device list and monitor readout
• Reopen the menu and confirm the name persists after restart
• Open "Rename Device…" again, clear the name, confirm it reverts to the auto-detected name
• Confirm "Rename Device…" is greyed out when no device is selected

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