Tag beamer frame titles as H1 headings for PDF accessibility

Patches the frametitle template to emit tagpdf H1 structure tags,
addressing accessibility reports about missing headings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

slides_template/example.tex

@@ -10,6 +10,15 @@
 \usepackage{tagpdf}
 \tagpdfsetup{activate-all}
 
+% Tag frame titles as H1 headings for PDF accessibility
+\addtobeamertemplate{frametitle}{%
+  \tagstructbegin{tag=H1}%
+  \tagmcbegin{tag=H1}%
+}{%
+  \tagmcend%
+  \tagstructend%
+}
+
 \usepackage{pgfpages}
 \input{notes.config}