[Hackathon demo] Present the not-found navigation as a clear primary action

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

## Problem

The not-found screen offers Go to trails as plain text, so the recovery action is easy to miss.

## Scope

Style the existing Link in app/+not-found.tsx as a compact primary action using current theme and spacing tokens. Keep the route and copy unchanged.

## Acceptance criteria

- Go to trails has a clear button-like visual treatment.
- The action exposes an appropriate accessibility role and label.
- Pressing it still returns to the Trails tab.
- Light and dark appearances remain readable.

## Validation

Follow docs/mobile-agent-loop.md.

Before: open an unknown route and capture the current screen plus action attributes.

After: open the same route, capture the updated screen, prove the action label with agent-device, and press it to verify navigation.

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