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_api.py

@@ -1,11 +1,18 @@
 """Tests for API endpoints (public + authenticated)."""
 
+import os
+import sys
+
+import pytest
 from fastapi.testclient import TestClient
 
 from wled_controller.main import app
 from wled_controller import __version__
 from wled_controller.config import get_config
 
+_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)")
+
 client = TestClient(app)
 
 # Build auth header from the first configured API key
@@ -41,8 +48,9 @@ def test_version_endpoint():
     assert data["api_version"] == "v1"
 
 
+@requires_display
 def test_get_displays():
-    """Test get displays endpoint (requires auth)."""
+    """Test get displays endpoint (requires auth and a real display)."""
     response = client.get("/api/v1/config/displays", headers=AUTH_HEADERS)
     assert response.status_code == 200
     data = response.json()