Hardcoded BACKEND_BASE_URL is emulator-only; should be a build property

[dzuluaga]

Problem

app/androidApp/build.gradle.kts hard-codes:
```kotlin
buildConfigField("String", "BACKEND_BASE_URL", "\"http://10.0.2.2:8000\\\"\")
```

10.0.2.2 is the Android emulator loopback to host. On a physical device it isn't routable to the dev machine. Every contributor hits this on first run.

Workaround (currently in README)

For physical device + USB:
```
adb reverse tcp:8000 tcp:8000
```
…and edit the buildConfigField to localhost:8000 before installing. Edit isn't documented.

For LAN: use host machine's LAN IP and ensure backend binds with --host 0.0.0.0 (now in README).

Proposed fix

Make it a Gradle property with a sensible default:
```kotlin
val backendUrl = (project.findProperty("backendUrl") as? String) ?: "http://10.0.2.2:8000\"
buildConfigField("String", "BACKEND_BASE_URL", "\"$backendUrl\"")
```

Then: ./gradlew installDebug -PbackendUrl=http://localhost:8000 for physical device, or -PbackendUrl=http://192.168.x.x:8000 for LAN.

Document both modes in README.

Caught by

PR #4 rebase validation — physical device couldn't reach backend until I patched this locally.