feat(home-assistant): enable ZHA for ZBT-2 Zigbee dongle

Adds the `zha` extra component so Home Assistant can drive the
Nabu Casa Connect ZBT-2 radio, and puts the `hass` service user in
`dialout` so it can open `/dev/serial/by-id/usb-Nabu_Casa_..._ZBT-2_*`.

Pairing is then handled through the standard ZHA wizard in the HA UI.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

modules/environments/home-assistant/default.nix

@@ -24,6 +24,7 @@ in
       extraComponents = [
         "matter"
         "mobile_app"
+        "zha"
       ];
     };
     services.home-assistant.config = {
@@ -32,6 +33,9 @@ in
       mobile_app = {};
     };
 
+    # Allow the `hass` service user to open the ZBT-2 USB-CDC serial endpoint.
+    users.users.hass.extraGroups = [ "dialout" ];
+
     my.homepage.services = [
       {
         group = "Services";