[Hackathon demo] Label the Clear filters action for assistive technology

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

## Problem

On the Trails empty-results state, the Clear filters action has visible text but no explicit accessibility label. The agent-device tree should expose a stable, descriptive action name.

## Scope

Update only the Clear filters Pressable in app/(tabs)/index.tsx. Keep its visible text and behavior unchanged.

## Acceptance criteria

- The action exposes the accessibility label Clear all trail filters.
- Pressing it still clears search, region, and difficulty filters.
- No unrelated layout or filter changes.

## Validation

Follow docs/mobile-agent-loop.md.

Before: use agent-device to create an empty-results state, capture a screenshot, and record the current action attributes.

After: return to the same state, prove the new label with a targeted agent-device query, capture the matching screenshot, and verify the action clears the filters.

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