Move slide thumbnails into responsive side rail

[merval]

Motivation

• Provide a desktop/tablet layout where slide thumbnails become a left-side, sticky navigation rail so thumbnails remain visible while scrolling notes.
• Keep the existing compact single-column thumbnail strip for narrow/mobile screens and ensure the layout adapts to short viewports.

Description

• Increased the desktop app container width to min(1280px, calc(100% - (var(--page-gutter) * 2))) to make room for a notes column and thumbnail rail in propresenter_notes/static/style.css.
• Added a @media (min-width: 901px) breakpoint that turns the .slideBrowser into a sticky left-side rail and switches the main layout to a two-column grid with a --thumbnail-sidebar-width CSS variable.
• Added an additional @media (min-width: 901px) and (max-height: 680px) rule to tighten spacing for short viewports and keep the rail usable.
• Preserved the existing single-column/mobile behavior under @media (max-width: 700px) so touch targets and bottom navigation remain unchanged.

Testing

• Ran the unit test suite with python -m unittest discover -s tests -v, which executed 59 tests and all passed (OK).
• Attempted automated screenshot capture with Playwright via npx --yes playwright install chromium, but the Playwright browser download was blocked by the environment (CDN returned 403), so browser-based UI screenshots could not be produced.
• Attempted to install system Chromium with apt-get install -y chromium, but the installed chromium-browser requires snapd and the snapd path is not available in this environment, preventing a usable browser for further UI testing.

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Codex Task