[Bug] Annotation badge/composer from a slide persists when navigating to the next slide in HTML slide decks

### What happened?

**Description**

When annotating a self-contained HTML slide deck (single HTML file with embedded JS driving fullscreen slide navigation via arrow keys/swipe/click, no
server/build step) using `plannotator annotate .html`, leaving an annotation on a slide and then navigating to the next slide does not clear the annotation UI. The badge and/or comment composer from the previous slide remains
visible, overlaid on top of the new slide's content, instead of being hidden or scoped to the slide it was left on.

**Steps to reproduce**

1. Open a self-contained HTML slide deck that implements fullscreen slide navigation via JS (click, arrow keys, swipe).
2. Run `plannotator annotate deck.html`.
3. In Pinpoint mode, click an element on the current slide to leave an annotation — the numbered badge / comment composer appears as expected.
4. Navigate to the next slide (arrow key, swipe, or click, depending on how the deck's own JS drives navigation).
5. Observe that the annotation badge/composer from the previous slide is still visible, superimposed on the new slide, instead of disappearing along with
the slide it was attached to.

### Plannotator version

0.27.5

### OS

Linux

### Agent

Claude Code

### Where did it happen?

Annotate

Start by reproducing the issue with a self-contained deck.html through `plannotator annotate deck.html`, using the listed slide navigation methods. Trace the annotation badge and comment composer while moving between slides; done means both are hidden or correctly scoped after navigation to the next slide.

Written by the indexing model from the issue text.