fix: make the computer button label match its status

[asasemahmed]

What changed

The header's computer button always had the accessibility label "Agent computer — take control", even when it showed "offline" or "ready", so screen readers announced the wrong state. The visible status and the label now come from one computerStatus() helper.

Verification

• New apps/mobile/test/computer-status.test.ts covers offline, ready and take control.
• Web app, sample workspace: with the worker stopped the button's accessible name is "Agent computer — offline"; with it running, "Agent computer — ready". No visual change.
• pnpm typecheck, biome ci on the changed files, pnpm test (206 passed, 0 failed, 1 skipped) and pnpm build:web pass. Not checked with VoiceOver/TalkBack on a device.