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>

Makefile

@@ -1,10 +1,12 @@
-PANDOC     = pandoc
-PDF_ENGINE = weasyprint
-TEMPLATE   = template.html
-CSS        = style.css
-PARTNERS   = $(patsubst %.md,%.pdf,$(wildcard partners/*.md))
+PANDOC      = pandoc
+PDF_ENGINE  = weasyprint
+TEMPLATE    = template.html
+CSS         = style.css
+PPTX_REF    = reference.pptx
+PARTNERS    = $(patsubst %.md,%.pdf,$(wildcard partners/*.md))
+PPTX_PARTNERS = $(patsubst %.md,%.pptx,$(wildcard partners/*.md))
 
-.PHONY: all clean
+.PHONY: all slides clean
 
 all: template.pdf $(PARTNERS)
 
@@ -13,8 +15,16 @@ all: template.pdf $(PARTNERS)
 %.pdf: %.md $(TEMPLATE) $(CSS)
 	$(PANDOC) $< -o $@ --template=$(TEMPLATE) --pdf-engine=$(PDF_ENGINE) --standalone
 
+# Slide deck for screen-share recordings. pptx ignores style.css -- its look
+# (colors/fonts) comes from reference.pptx's slide master instead, so that's
+# the file to edit if the slide theme needs to change.
+slides: template.pptx $(PPTX_PARTNERS)
+
+%.pptx: %.md $(PPTX_REF)
+	$(PANDOC) $< -o $@ --reference-doc=$(PPTX_REF)
+
 clean:
-	rm -f template.pdf partners/*.pdf
+	rm -f template.pdf template.pptx partners/*.pdf partners/*.pptx
 
 partnership: template.md
 	cp template.md partners/$(SLUG).md