Show the subtitle on the title page

Ports the fix now proposed upstream: trantor's title page never printed
\subtitle, so a deck that set one silently lost it.

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

slides_template/trantor.sty

@@ -247,6 +247,10 @@
     \begin{minipage}{0.82\paperwidth}
       \raggedright
       {\color{TrantorDark}\headerfont\Large\bfseries \@title\par}%
+      \ifx\@subtitle\empty\else
+        \vspace{0.35em}%
+        {\color{TrantorDark}\headerfont\large \@subtitle\par}%
+      \fi
       \vspace{0.5em}%
       {\color{TrantorAccent}\rule{\linewidth}{0.8pt}\par}%
       \vspace{1.3em}%