[Audit][P1] Restore LearnDash progression and certificate resilience

## Context

Topicless lessons cannot render their completion control, and certificate views assume an assigned certificate post always exists.

Audit findings: 15 and 16.

## Scope

- Correct completion behavior for lessons with and without topics.
- Preserve certificate access on completed lessons where appropriate.
- Guard missing, deleted, or unpublished certificates in course and My Courses views.
- Guard completion-related DOM listeners.

## Done when

- Topicless lessons can be completed.
- Lessons with topics follow documented LearnDash rules.
- Missing certificates produce a controlled state rather than warnings or broken output.
- Progress and certificate scenarios have regression coverage.