slides: fix title page spacing, enable LIP6 and Sorbonne

- Reserve a left margin in the partner logo grid so a full grid stays
  clear of the affiliations.
- Let the space above the title shrink when the title page overflows,
  so the logos keep a bottom margin equal to the side margins.
- Enable the LIP6 and Sorbonne Université partners in the example deck.
- Note that author and institute blocks must stay plain: table and
  flushleft wrappers add vertical space.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Slides/slides.cls

@@ -195,8 +195,10 @@
 \setbeamertemplate{title page}{
   % Use the theme's margins rather than a full-paper-height box, which placed
   % the footer almost on the page edge. Stretch only the space above the footer.
+  % If the content is too tall, shrink the top space so the footer keeps its
+  % bottom margin and the title moves up instead.
   \begin{minipage}[t][\dimexpr\paperheight-2\beamer@leftmargin\relax][s]{\textwidth}
-    \vspace*{.04\paperheight}
+    \vspace*{\dimexpr .04\paperheight\relax minus 1fill}
     \ifx\inserttitle\@empty\else\usebeamertemplate*{title}\fi
     \ifx\insertsubtitle\@empty\else\usebeamertemplate*{subtitle}\fi
     \vspace{.2cm}
@@ -210,7 +212,7 @@
         \column{0.44\linewidth}
         \insertpartnerlogos
     \end{columns}
-    \vspace*{.02\paperheight}
+    \vspace*{0pt}
   \end{minipage}
 }
 \makeatother