Add offline presentation cache fallback

[merval]

Motivation

• Ensure the speaker view has the full presentation slides, thumbnails and notes available even if the connection to ProPresenter is lost so the speaker can continue advancing slides with access to notes.
• Provide a graceful offline UX by falling back to previously cached presentations and by letting local navigation advance notes when trigger calls fail.

Description

• Add browser storage keys and helpers in propresenter_notes/static/app.js to persist the presentation list and per-presentation cached slide payloads (presentationsStorageKey, presentationCacheStoragePrefix, readStoredJson, writeStoredJson, readStoredPresentationCache, and writeStoredPresentationCache).
• Persist the presentation list after successful /api/presentations and fall back to the saved list when the live request fails, showing an explanatory cache status message.
• Save the server-provided presentation cache from /api/presentation-cache/:id to browser storage (stripping the large raw payload) and fall back to that cached slide set when live caching fails, with the UI entering an offline mode message.
• Make triggerSlide resilient by catching failures from the trigger endpoint and returning false so navigation UI (Previous/Next, keyboard, swipe, thumbnails) still advances notes locally from the cached slides while showing an offline status.
• Wrap initial loading of presentations/cache in try/catch so startup surfaces a helpful offline message instead of failing silently and document the offline behavior in README.md.

Testing

• Static JS syntax check with node --check propresenter_notes/static/app.js succeeded.
• Ran the full Python unit test suite with python -m unittest discover -s tests -v, which ran 59 tests and all passed (OK).

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Codex Task