fix: persist car-connected flag so trip building works across reboots (BUG-16)

connect_obd() set _car_ever_connected without a global declaration, so the
module-level flag was never set and was_car_ever_connected() always returned
False — local trip building has been silently blocked since BUG-10 landed.
Also persist the flag to the settings table so a drive followed by a reboot
still builds trips, and clean up dead code in was_car_ever_connected().

tests/test_app_obd.py

@@ -0,0 +1,30 @@
+import unittest
+from unittest.mock import patch
+
+from modules import app_obd
+
+
+class CarConnectedFlagTest(unittest.TestCase):
+    def setUp(self):
+        app_obd._car_ever_connected = False
+
+    def tearDown(self):
+        app_obd._car_ever_connected = False
+
+    def test_was_ever_connected_reads_persisted_flag(self):
+        with patch("modules.app_sql.get_setting", return_value="1"):
+            self.assertTrue(app_obd.was_car_ever_connected())
+            self.assertTrue(app_obd._car_ever_connected)
+
+    def test_was_ever_connected_defaults_false(self):
+        with patch("modules.app_sql.get_setting", return_value="0"):
+            self.assertFalse(app_obd.was_car_ever_connected())
+
+    def test_persist_car_connected_writes_setting(self):
+        with patch("modules.app_sql.set_setting") as mock_set:
+            app_obd._persist_car_connected()
+            mock_set.assert_called_once_with("car_ever_connected", "1")
+
+
+if __name__ == "__main__":
+    unittest.main()