Add pptx slide-deck build target for screen-share recordings

Same markdown now also builds a PowerPoint deck (make slides /
make template.pptx) alongside the existing PDF, for recording
walkthrough videos. Styling comes from reference.pptx since pandoc's
pptx writer ignores style.css; its theme was hand-tuned to match the
PDF's dark palette and fonts.

Restructure the closing block into a two-column div so the author
photo survives in slide exports (pandoc drops images sharing a plain
div with paragraph text), with a CSS override so the PDF keeps its
original centered layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

style.css

@@ -74,6 +74,13 @@ img {
   text-align: center;
 }
 
+/* .closing is also a pandoc "columns" div (photo | text) so its image
+   survives in slide exports -- neutralize the side-by-side layout here so
+   the PDF still renders it as a centered stack. */
+.closing .column {
+  width: auto !important;
+}
+
 .closing .author-photo {
   width: 100px;
   border-radius: 50%;