feat v1.29.0: 'new device' flag with a red dot

- auto devices/light groups appearing after install get a server-side 'new'
  flag (settings.new_device_ids) and a red dot top-right of the icon; opening
  the device's editor clears it for every client
- known_devices baseline seeded silently on first run — upgrades never flood
  the plan with dots; hand-made markers never flagged
- pure diffNewDevices + unit tests; backend schema; smoke_new_device.mjs;
  TESTING.md row

custom_components/houseplan/validation.py

@@ -158,6 +158,8 @@ CONFIG_SCHEMA = vol.Schema(
         vol.Optional("markers", default=list): [MARKER_SCHEMA],
         vol.Optional("settings", default=dict): vol.Schema(
             {
+                vol.Optional("known_devices"): [str],
+                vol.Optional("new_device_ids"): [str],
                 vol.Optional("fill_colors"): vol.Schema(
                     {
                         str: vol.Schema(