Presentation download menu: standalone HTML + faithful slide PDF (decktape)

In RevealJS mode the download button becomes a 2-choice menu:

- Standalone HTML: a one-off compile with embed-resources (chalkboard and other
  runtime-only plugins are dropped, since they don't survive self-containment),
  yielding a single portable .html.
- Slide PDF: render the deck, then print it with decktape (headless Chromium)
  to a faithful one-slide-per-page PDF.

Implementation:
- Dockerfile-base: install decktape + headless Chromium (open-source; deps via
  playwright install-deps for Ubuntu-Noble correctness). Base-only change.
- QuartoRunner honours options.exportMode ('html-standalone' | 'pdf-slides');
  exportMode is threaded web ClsiManager -> CLSI RequestParser -> CompileManager
  -> runner.
- New GET /project/:id/presentation-export/:format compiles in the matching
  export mode and streams the result as a download (PresentationExportController,
  reusing ClsiManager.getOutputFileStream).
- pdf-hybrid-download-button shows the dropdown when the output is output.html;
  PDF/LaTeX projects keep the single download button.
- i18n: download_as_standalone_html / download_as_pdf_slides (en + fr +
  extracted-translations.json).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server-ce/Dockerfile-base

@@ -69,6 +69,24 @@ RUN mkdir -p /opt/quarto-extensions \
     \
 &&  chown -R www-data:www-data /opt/quarto-extensions
 
+# Install decktape + headless Chromium (for exporting RevealJS decks to PDF)
+# -----------------------------------------------------------------------
+# decktape drives a headless Chromium (via Puppeteer) to print the rendered
+# reveal.js slides to a faithful, one-slide-per-page PDF. Chromium is the
+# open-source engine (BSD); decktape is MIT, Puppeteer Apache-2.0 — all
+# permissive and AGPL-compatible. They are invoked as a separate process
+# (QuartoRunner runs `decktape ...`), never linked into the app.
+#
+# Puppeteer downloads its Chromium into PUPPETEER_CACHE_DIR during the global
+# install; we put it in a world-readable /opt path so the www-data runtime user
+# can launch it. Playwright is used only as a robust, distro-aware installer for
+# Chromium's system libraries (handles Ubuntu Noble's t64 package renames).
+ENV PUPPETEER_CACHE_DIR=/opt/puppeteer
+RUN npm install -g decktape \
+&&  npx --yes playwright@latest install-deps chromium \
+&&  chmod -R a+rX /opt/puppeteer \
+&&  rm -rf /root/.npm /root/.cache
+
 # Install TeX Live (for compiling .tex projects with latexmk)
 # -----------------------------------------------------------------------
 # Verso compiles .qmd with Quarto and .tex with latexmk; both engines live