runtime: make Full Skin switching transactional

[rwang23]

Summary

• stage candidate Full Skin injection before replacing the current session
• roll back partial target injection and preserve the previous session on candidate failure
• expose healthy, recovering, degraded, and stopped monitor states in the runtime and bilingual UI

Verification

• npm run desktop:check
• npm run test:source
• node --check apps/theme-manager/src/renderer/app.js
• git diff --check

Independent acceptance review

Pass with risks: targeted Rust regressions prove rollback ordering and monitor-state transitions. A real isolated Stable/Beta package + CDP switch smoke remains pending, so this PR does not claim live-app verification.

Closes #7

[rwang23]

Superseded by #26, now merged into main at e2ed6e8. This branch head is already included in main; closing the draft avoids duplicate review and merge work.