Keep screen awake during conversion

[Alcray]

Summary

• Closes Is it possible to keep the user's computer from sleeping while document conversion is in progress? #16 by requesting a best-effort Screen Wake Lock while initial conversions and feedback re-runs are in progress.
• Releases the wake lock when conversion finishes, fails, or upload/feedback submission fails.
• Re-acquires the lock when the tab becomes visible again during an active conversion.

Test plan

• Linter: no issues in public/demo.html
• npm run typecheck currently fails on latest iris-implementation before this change: src/pipeline/regression.ts(51,11): Property 'reconciled' does not exist on type 'Fragment'
• Manual: start a conversion in a Wake Lock-capable browser and verify navigator.wakeLock is requested until completion