feat(offline): add manual connectivity verification

[rdlabo]

Summary

• add manual API reachability verification to the existing offline network/coordinator services
• share concurrent checks, apply an 8-second timeout, and bypass local read fallback
• order API reachability observations so stale probe failures cannot overwrite newer results
• cover success, HTTP errors, transport failures, timeout, retry, and single-flight behavior

Verification

• npm run lint
• npx ng test kit --watch=false (54 files, 1037 tests)
• npm run prebuild:kit
• upper-management review: approved after the observation-order fix
• third-party acceptance review: approved after the observation-order fix

Rollout

After merge, publish a patch release before updating the five consuming apps.

[devin-ai-integration]

Devin Review found 1 potential issue.