Scaffold backend gateway and integration docs

docs/SETUP.md

@@ -0,0 +1,35 @@
+# Setup
+
+## Backend Base URL
+
+Build-time (recommended):
+
+```bash
+flutter run --dart-define=BACKEND_BASE_URL=https://api.example.com
+```
+
+Runtime override (for future settings screens):
+
+```dart
+AppConfig.overrideBackendBaseUrl('https://staging.example.com');
+```
+
+## Run With Fake Gateway
+
+Use fake gateway for local/offline development:
+
+```bash
+flutter run --dart-define=USE_FAKE_BACKEND=true
+```
+
+## Run Tests
+
+```bash
+flutter test
+```
+
+## Generate Code
+
+```bash
+dart run build_runner build --delete-conflicting-outputs
+```