Reduce content-script watchdog polling cost

[bartekplus]

Summary

• replace unconditional 1s watchdog polling in content scripts
• schedule watchdog checks only on relevant events (navigation, lifecycle, focus, visibility, and root DOM changes)
• debounce event bursts by 250ms to avoid repeated checks
• keep existing hostname and DOM-root recovery behavior

Why

• content scripts run in all frames, so interval polling created steady background work in every frame, including idle/disabled contexts

Testing

• npm run build
• npm test -- tests/content_script.watchdog.test.ts
• npm test