fix: prevent pace values from displaying 60 seconds

[janicduplessis]

Description

formatPace floored the minute value before independently rounding the remaining seconds, so a remainder near 60 could render as 4:60. It now rounds the total pace seconds before splitting minutes and seconds, allowing the minute to carry normally without affecting the other formatters.

Test plan

• node --test src/lib/format.test.mjs passes four cases covering rollover, zero padding, metric and imperial suffixes, and an unchanged normal pace.
• Opened seeded hike Gorge Haut des Caps on a Stim-owned iOS simulator and verified its average pace on the same card before and after the fix.

Before

$ node -e "import('./src/lib/format.ts').then(({ formatPace }) => console.log(formatPace(299.6, 1000, 'metric')))"
4:60 /km

After

$ node -e "import('./src/lib/format.ts').then(({ formatPace }) => console.log(formatPace(299.6, 1000, 'metric')))"
5:00 /km

Closes #10