0.6.2: add Bluetooth panel

Adapter power switch, paired devices (connect/disconnect/forget), and
scan-for-new-devices with pairing -- BOS had bluez/bluez-utils packaged
and bread's own daemon already monitors Bluetooth events, but there was
no GUI to actually manage devices anywhere in the settings app.

Shells out to bluetoothctl's non-interactive mode, same "standard CLI,
no new dependency" choice as Network (nmcli) and Snapshots (snapper).
Pairing only covers "just works" SSP (most audio/HID devices) -- a
device requiring a PIN would need this app to register its own
bluetoothd agent, out of scope for now; it just fails with the reason
shown, same honest-failure pattern the Firewall fix added.

src/ui/sidebar.rs

@@ -33,6 +33,7 @@ const fn item_sub(
 pub const SYSTEM_ITEMS: &[SidebarItem] = &[
     item("network", "Network", "network-wireless-symbolic"),
     item_sub("breadcrumbs", "Wi-Fi Profiles", "breadcrumbs", "network-workgroup-symbolic"),
+    item("bluetooth", "Bluetooth", "bluetooth-symbolic"),
     item("firewall", "Firewall", "security-high-symbolic"),
     item("sound", "Sound", "audio-volume-high-symbolic"),
     item("power", "Power", "battery-good-symbolic"),