Introduces a new "group" device type that aggregates multiple physical
(or nested group) devices into one virtual device. Supports two modes:
- Sequence: LEDs concatenated end-to-end (led_count = sum of children)
- Independent: full pixel array resampled to each child independently
Includes cycle detection (DFS) to prevent circular group references,
delete protection for devices referenced by groups, recursive LED count
resolution for nested groups, and reorder controls (move up/down) for
child devices in the UI.
Backend: Device model, API schemas, GroupLEDClient, GroupDeviceProvider,
route validation, processing pipeline integration.
Frontend: type picker, child device picker with reorder, mode selector,
i18n (en/ru/zh), layers icon, CSS for group child rows.
Tests: 20 unit tests for cycle detection, LED count resolution, and
GroupLEDClient (sequence slicing, independent resampling, cleanup).