fix(ci): resolve Pillow JPEG KeyError and cairosvg missing dep

convert.py: Pillow's PDF writer requires libjpeg for RGB images, which
is not available in the CI Python environment. Replace all Pillow PDF
saves with _save_images_as_pdf(), which writes pages as lossless PNGs
and assembles them with qpdf — no JPEG dependency needed.

build.yml: install mkdocs-material[imaging] instead of mkdocs-material
to satisfy the cairosvg dependency required by the social plugin.

Signed-off-by: nopeitsnothing <no@anonymousplanet.org>

.github/workflows/build.yml

@@ -42,7 +42,7 @@ jobs:
           python-version: "3.13"
 
       - name: 📦 Install Python dependencies
-        run: pip install mkdocs-material pillow numpy
+        run: pip install "mkdocs-material[imaging]" pillow numpy
 
       - name: 🖼️ Install poppler and qpdf
         run: |