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/src/wled_controller/api/routes/system.py

@@ -55,7 +55,7 @@ logger = get_logger(__name__)
 psutil.cpu_percent(interval=None)
 
 # GPU monitoring (initialized once in utils.gpu, shared with metrics_history)
-from wled_controller.utils.gpu import nvml_available as _nvml_available, nvml as _nvml, nvml_handle as _nvml_handle
+from wled_controller.utils.gpu import nvml_available as _nvml_available, nvml as _nvml, nvml_handle as _nvml_handle  # noqa: E402
 
 
 def _get_cpu_name() -> str | None: