fix: lazy-import tkinter to fix CI on headless Linux

screen_overlay.py imported tkinter at module level, which cascaded
through processor_manager → every test touching the app on CI where
libtk8.6.so is unavailable. Move to TYPE_CHECKING + runtime lazy
import so the overlay module loads cleanly on headless systems.

Also fix test_processor_manager.py to use ProcessorDependencies().

server/tests/test_processor_manager.py

@@ -2,7 +2,7 @@
 
 import pytest
 
-from wled_controller.core.processing.processor_manager import ProcessorManager
+from wled_controller.core.processing.processor_manager import ProcessorDependencies, ProcessorManager
 from wled_controller.core.processing.processing_settings import ProcessingSettings
 from wled_controller.core.capture.calibration import create_default_calibration
 
@@ -33,12 +33,12 @@ def mock_wled_responses():
 @pytest.fixture
 def processor_manager():
     """Provide processor manager instance."""
-    return ProcessorManager()
+    return ProcessorManager(deps=ProcessorDependencies())
 
 
 def test_processor_manager_init():
     """Test processor manager initialization."""
-    manager = ProcessorManager()
+    manager = ProcessorManager(deps=ProcessorDependencies())
     assert manager is not None
     assert manager.get_all_devices() == []