fix: skip display-dependent tests on headless CI

Tests that call get_available_displays() or capture_display() require
a real display ($DISPLAY on Linux, always available on Windows/macOS).
Mark them with @requires_display to skip on headless CI instead of
failing with "$DISPLAY not set".

Affects: test_screen_capture.py (4 tests), test_api.py (1 test).

server/tests/test_screen_capture.py

@@ -1,5 +1,8 @@
 """Tests for screen capture functionality."""
 
+import os
+import sys
+
 import numpy as np
 import pytest
 
@@ -14,7 +17,12 @@ from wled_controller.core.capture.screen_capture import (
     ScreenCapture,
 )
 
+# Skip tests that require a real display on headless CI
+_has_display = bool(os.environ.get("DISPLAY") or sys.platform == "win32" or sys.platform == "darwin")
+requires_display = pytest.mark.skipif(not _has_display, reason="No display available (headless CI)")
 
+
+@requires_display
 def test_get_available_displays():
     """Test getting available displays."""
     displays = get_available_displays()
@@ -32,6 +40,7 @@ def test_get_available_displays():
     assert display.height > 0
 
 
+@requires_display
 def test_capture_display():
     """Test capturing a display."""
     # Capture the first display
@@ -46,6 +55,7 @@ def test_capture_display():
     assert capture.image.shape == (capture.height, capture.width, 3)
 
 
+@requires_display
 def test_capture_display_invalid_index():
     """Test capturing with invalid display index."""
     with pytest.raises(ValueError):
@@ -197,6 +207,7 @@ def test_calculate_color_empty_pixels():
     assert calculate_dominant_color(empty_pixels) == (0, 0, 0)
 
 
+@requires_display
 def test_end_to_end_screen_capture():
     """Test complete screen capture workflow."""
     # Get available displays