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_api.py

@@ -1,6 +1,5 @@
 """Tests for API endpoints (public + authenticated)."""
 
-import pytest
 from fastapi.testclient import TestClient
 
 from wled_controller.main import app