docs(readme): expand Setup & Building quick start for all platforms

Restructure the quick start with Web, Android, and iOS subheadings; put
each npm command in its own code block; fold the test-page step into the
Web section. Document Android (build:android:test:run + ADB, link to
BUILDING.md) and iOS (build:ios:studio + Xcode prerequisites).

README.md

@@ -15,10 +15,31 @@ Quick start:
 
 ```bash
 npm install
+```
+
+### Web
+
+```bash
 npm run build:web:dev
 ```
 
-To be able to take action on the platform: go to [the test page](http://localhost:8080/test) and click "Become User 0".
+Then go to [the test page](http://localhost:8080/test) and click "Become User 0" to take action on the platform.
+
+### Android
+
+```bash
+npm run build:android:test:run
+```
+
+Assumes ADB is installed; see [Android Build](BUILDING.md#android-build) for SDK, emulator, and `PATH` setup.
+
+### iOS
+
+```bash
+npm run build:ios:studio
+```
+
+Assumes Xcode and Xcode Command Line Tools are installed.
 
 See [BUILDING.md](BUILDING.md) for comprehensive build instructions for all platforms (Web, Electron, iOS, Android, Docker).