Keep offline slide navigation responsive

[merval]

Motivation

• Offline and slow API conditions caused the visible speaker notes to wait for ProPresenter API responses, making slide navigation feel sluggish and tied to network availability.

Description

• Added local queueing state with pendingSlideTrigger and slideTriggerInFlight to serialize and track background slide trigger work in propresenter_notes/static/app.js.
• Replaced immediate API calls from thumbnail taps and previous/next with a local-first UX that calls showSlide(...) immediately and enqueues the actual ProPresenter trigger via queueSlideTrigger(...).
• Implemented queueSlideTrigger, flushSlideTriggerQueue, and sendSlideTrigger to coalesce rapid navigation to the latest requested slide and avoid out-of-order API requests.
• Simplified trigger(direction) to advance notes locally and queue the API call asynchronously so speaker pace is not blocked by the ProPresenter API.

Testing

• Ran node --check propresenter_notes/static/app.js to validate the script syntax without errors, which succeeded.
• Ran the full Python unit test suite with python -m unittest discover -s tests -v, which executed 59 tests and completed successfully (OK).

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Codex Task