[Hackathon demo] Prevent pace values from displaying 60 seconds

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

## Problem

formatPace rounds the seconds component independently. Values near the next minute can render as 4:60 /km instead of carrying into 5:00 /km.

## Scope

Fix the rollover in src/lib/format.ts and add focused coverage using the repository's existing test approach. Do not change distance, elevation, or duration formatting.

## Acceptance criteria

- Rounded 60 seconds carries into the minute value.
- Seconds remain zero-padded.
- Metric and imperial suffixes remain correct.
- Existing normal pace values do not change.

## Validation

Follow docs/mobile-agent-loop.md.

Before: include a minimal command or test demonstrating the failing 4:60-style output.

After: run the same input and show the corrected 5:00-style output. If a seeded hike can reproduce it in the UI, include matching agent-device screenshots; otherwise state why visual evidence does not apply.

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