Force simulate mode when OSM OAuth secrets are missing

Preview/PR builds don't have access to GitHub Secrets, so the OAuth
client IDs are empty. Previously this caused a runtime crash from
keys.dart throwing on empty values. Now we detect missing secrets
and force simulate mode, which already fully supports fake auth
and uploads.

Also fixes a latent bug where forceLogin() would crash with
LateInitializationError in simulate mode since _helper is never
initialized when OAuth setup is skipped.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/state/settings_state_test.dart

@@ -0,0 +1,73 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:deflockapp/state/settings_state.dart';
+import 'package:deflockapp/keys.dart';
+
+void main() {
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+  });
+
+  group('kHasOsmSecrets (no --dart-define)', () {
+    test('is false when built without secrets', () {
+      expect(kHasOsmSecrets, isFalse);
+    });
+
+    test('client ID getters return empty strings instead of throwing', () {
+      expect(kOsmProdClientId, isEmpty);
+      expect(kOsmSandboxClientId, isEmpty);
+    });
+  });
+
+  group('SettingsState without secrets', () {
+    test('defaults to simulate mode', () {
+      final state = SettingsState();
+      expect(state.uploadMode, UploadMode.simulate);
+    });
+
+    test('init() forces simulate even if prefs has production stored', () async {
+      SharedPreferences.setMockInitialValues({
+        'upload_mode': UploadMode.production.index,
+      });
+
+      final state = SettingsState();
+      await state.init();
+
+      expect(state.uploadMode, UploadMode.simulate);
+
+      // Verify it persisted the override
+      final prefs = await SharedPreferences.getInstance();
+      expect(prefs.getInt('upload_mode'), UploadMode.simulate.index);
+    });
+
+    test('init() forces simulate even if prefs has sandbox stored', () async {
+      SharedPreferences.setMockInitialValues({
+        'upload_mode': UploadMode.sandbox.index,
+      });
+
+      final state = SettingsState();
+      await state.init();
+
+      expect(state.uploadMode, UploadMode.simulate);
+    });
+
+    test('init() keeps simulate if already simulate', () async {
+      SharedPreferences.setMockInitialValues({
+        'upload_mode': UploadMode.simulate.index,
+      });
+
+      final state = SettingsState();
+      await state.init();
+
+      expect(state.uploadMode, UploadMode.simulate);
+    });
+
+    test('setUploadMode() allows simulate', () async {
+      final state = SettingsState();
+      await state.setUploadMode(UploadMode.simulate);
+
+      expect(state.uploadMode, UploadMode.simulate);
+    });
+  });
+}