Split presentation selection into Library + Presentation dropdowns

[merval]

Motivation

• The flat presentation list across all libraries becomes too long to browse comfortably, so users need a way to narrow presentations by library first.

Description

• Added a Library dropdown to the UI and updated the template to render the two-select layout (propresenter_notes/templates/index.html).
• Implemented library-aware client-side logic in propresenter_notes/static/app.js, including uniqueLibraries, renderLibraryOptions, renderPresentationOptions, persisted libraryId in local storage, and updated selection/change handlers to cascade and reload caches accordingly.
• Updated propresenter_notes/static/style.css with a .selectGroup grid and responsive adjustments so the two selects appear side-by-side on wide screens and stack on mobile.

Testing

• Ran a syntax check with node --check propresenter_notes/static/app.js, which passed.
• Ran the Python test suite with python -m pytest, which passed (59 passed).
• Attempted a Playwright screenshot to visually verify the new controls, but browser installation was blocked by the environment (download returned 403 Domain forbidden).

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Codex Task