[Hackathon demo] Give the trail detail hero image a useful accessibility label

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

## Problem

The hero image on a trail detail screen has no useful accessibility description.

## Scope

Update the hero Image in app/trail/[id].tsx only. Derive a short label from the trail name; do not change the image source, transition, or layout.

## Acceptance criteria

- The hero image exposes a label in the form Photo of Trail Name.
- The label uses the current trail name.
- The visual layout is unchanged.

## Validation

Follow docs/mobile-agent-loop.md.

Before: open one trail, capture the screen, and record the hero image attributes with agent-device.

After: open the same trail, prove the new label with agent-device, and capture the same screen.

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