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_config.py

@@ -1,7 +1,5 @@
 """Tests for configuration management."""
 
-import os
-from pathlib import Path
 
 import pytest
 import yaml
@@ -9,13 +7,8 @@ import yaml
 from wled_controller.config import (
     Config,
     ServerConfig,
-    StorageConfig,
-    AuthConfig,
-    MQTTConfig,
-    LoggingConfig,
     get_config,
     reload_config,
-    is_demo_mode,
 )