Add unit tests for `usageLevel` threshold classifier

[tsouth89]

Context

usageLevel(pct, exhausted) in apps/desktop-tauri/src/surfaces/tray/UsageBar.tsx:11 maps a percentage into normal | high | critical | exhausted (thresholds at 70 and 90, with exhausted taking priority). Pure function, no test.

Task

Export usageLevel and add unit tests (a new UsageBar.test.tsx can hold both this and the ARIA test if that issue is done together).

Acceptance criteria

• Covers each band and both sides of the boundaries: 0, 69, 70, 89, 90, 100.
• Covers exhausted = true overriding a low percentage.
• Uses the existing vitest setup.

How to test

pnpm --dir apps/desktop-tauri test

Pointers

• Function: UsageBar.tsx:11