feat(desktop): add prevent sleep preference

[bobleer]

Summary

• add a persisted app.prevent_sleep preference that defaults to off
• keep the local computer awake for the full BitFun desktop process lifetime when enabled, independent of agent task activity
• apply config imports/reloads, keep peer-device control local, and expose the setting under Basics > Power
• add English, Simplified Chinese, and Traditional Chinese copy plus focused backend/frontend tests

The inhibitor prevents idle system sleep only. Display timeout, explicit sleep, and laptop lid behavior remain governed by the operating system.

Verification

• cargo check -p bitfun-desktop
• cargo test -p bitfun-desktop remote_workspace_policy --lib
• cargo test -p bitfun-desktop sleep_prevention --lib
• cargo test -p bitfun-core prevent_sleep_defaults_to_disabled --lib
• pnpm --dir src/web-ui run test:run src/infrastructure/api/service-api/SystemAPI.test.ts src/infrastructure/api/adapters/peer-device-adapter.test.ts
• pnpm run type-check:web
• pnpm run i18n:audit