Fix undo for completed polygons

[pankajarm]

Summary

• fix Undo so it removes the last completed polygon when there are no in-progress points
• clear JSON output when all points are removed
• update Clear All to target the existing coordinate output elements

Fixes #23.

Testing

• node --check script.js

[pankajarm]

Quick note on the failing check: this appears unrelated to the script.js changes in this PR.

The failed job is the repository's Welcome WorkFlow (actions/first-interaction@v1.1.1) on pull_request_target. The job log shows the workflow token only has read permissions and fails with:

Resource not accessible by integration

Local validation for this PR still passes:

node --check script.js

If helpful, the welcome workflow likely needs explicit write permissions in the base repo workflow, e.g. issues: write / pull-requests: write, or the failed welcome-bot check can be ignored for this PR since the Socket checks passed and no dependency files changed.