[Hackathon demo] Show the native build number in Settings

Hackathon demo ticket. This is intentionally small and self-contained.

## Problem

Settings shows the marketing version only, which makes it harder to tell two local native builds apart during QA.

## Scope

Update the About version value in app/(tabs)/settings.tsx using Expo Constants. Show the native build version when available and retain the current version-only fallback.

## Acceptance criteria

- Settings displays Version (build) when a native build version exists.
- It still displays Version when the native value is unavailable.
- No new dependency or settings row is added.

## Validation

Follow docs/mobile-agent-loop.md.

Before: capture the About section and read its version value with agent-device.

After: return to the same section, prove the full version and build text with agent-device, and capture the matching screenshot.

The PR description must include clearly labeled Before and After evidence.