fix: resolve all 153 ruff lint errors for CI

Auto-fixed 138 unused imports and f-string issues. Manually fixed:
ambiguous variable names (l→layer), availability-check imports using
importlib.util.find_spec, unused Color import, ImagePool forward ref
via TYPE_CHECKING, multi-statement semicolons, and E402 suppression.

server/tests/test_device_store.py

@@ -1,7 +1,6 @@
 """Tests for device storage."""
 
 import pytest
-from pathlib import Path
 
 from wled_controller.storage.device_store import Device, DeviceStore