Add Server Buddy desktop app with local Linux assessment and reporting

tests/test_startup.py

@@ -0,0 +1,17 @@
+import unittest
+from unittest.mock import patch
+from server_buddy.core.startup import desktop_error
+
+class StartupTests(unittest.TestCase):
+    @patch('server_buddy.core.startup.sys.platform', 'linux')
+    @patch.dict('os.environ', {'DISPLAY': ':0', 'QT_QPA_PLATFORM': 'xcb'}, clear=True)
+    @patch('server_buddy.core.startup.ctypes.CDLL', side_effect=OSError)
+    def test_missing_xcb_returns_install_instruction(self, loader):
+        self.assertIn('sudo apt install libxcb-cursor0', desktop_error())
+
+    @patch('server_buddy.core.startup.sys.platform', 'linux')
+    @patch.dict('os.environ', {'QT_QPA_PLATFORM': 'offscreen'}, clear=True)
+    @patch('server_buddy.core.startup.ctypes.CDLL')
+    def test_offscreen_does_not_require_xcb(self, loader):
+        self.assertIsNone(desktop_error())
+        loader.assert_not_called()