Add unit tests for the pace duration formatter (formatEta)

[tsouth89]

Context

formatEta(seconds) in apps/desktop-tauri/src/surfaces/settings/providers/sections/PaceSection.tsx:44 renders a seconds value as a compact m/h/d string (<60m → minutes, <24h → hours, else days). It is pure and has no test file.

Task

Add unit tests for formatEta. The function is currently module-private — export it (export function formatEta) so it can be imported directly by a new PaceSection.test.tsx (or a small PaceSection.formatEta.test.ts).

Acceptance criteria

• Covers the boundaries: 0, 59s, exactly 60s (→ 1m), 59m, 60m (→ 1h), 23h59m, 24h (→ 1d), and a multi-day value.
• Covers negative input (function clamps minutes at 0).
• Uses the existing vitest setup; no app launch.

How to test

pnpm --dir apps/desktop-tauri test

Pointers

• Function: PaceSection.tsx:44
• Sibling tests to copy the pattern from live under apps/desktop-tauri/src/lib/*.test.ts.