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

src/i18n/en.json

@@ -244,5 +244,6 @@
   "mode.plan": "Plan",
   "mode.devices": "Devices",
   "display.value": "Value instead of an icon",
-  "marker.subarea": "no area, manual"
+  "marker.subarea": "no area, manual",
+  "device.new": "New device — open its editor to dismiss"
 }