fix(annotator): isolate active threads per pane in side-by-side preview

[zhirongwang]

Summary

• In side-by-side compare, both annotator instances share one event manager, so activating a thread in one pane broadcast the id to both. The pane that did not own the annotation stored the foreign id, rendered an empty "Add a comment" composer, and echoing a deselect back to the host cancelled the real selection.
• BaseAnnotator now ignores activation ids that are not in its store (once annotations are loaded) and closes its own thread with a local clear that does not notify the host.
• PopupV2 renders nothing when the active annotation has no messages, instead of a bare composer.

Deep links still work: activation that arrives before annotations load is stored as before. Single-annotator behavior is unchanged.

Test plan

• Open a PDF in side-by-side compare with annotations on both versions
• Click a thread in the left pane: only that pane opens a thread; the right pane does not show a second empty composer
• Click a thread in the right pane: same, no leftover composer on the left
• Click away to close a thread: host still deselects as before
• Deep-link / reload with an annotation id in the URL: the thread still opens after annotations load
• Single-pane preview: opening, closing, and creating threads behaves as before

[JChan106]

lgtm