fix: set fixed slide dimensions and scaling for consistent sizing

Set 1280x720 canvas with margin 0.08 and scale range 0.2–2.0
so all slides render at the same size regardless of content.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

index.html

@@ -411,6 +411,11 @@ class webserver {
   });
 
   Reveal.initialize({
+    width: 1280,
+    height: 720,
+    margin: 0.08,
+    minScale: 0.2,
+    maxScale: 2.0,
     hash: true,
     slideNumber: 'c/t',
     transition: 'slide',