apply responsive image sizing globally to prevent mobile overflow

Graphviz diagrams are rendered/injected as plain `<img>` elements outside the
banner container, and on mobile they were causing horizontal page overflow.

The previous responsive image rule was scoped to `.banner img`, so those
non-banner images could exceed viewport width and trigger horizontal scrolling.

Move `max-width: 100%` and `height: auto` from `.banner img` to a global `img`
rule. This sets a site-wide default policy that images should fit their
container, while still preserving aspect ratio.

This immediately fixes horizontal overflow for Graphviz images and also provides
the same protection for future non-banner images by default.

Author: ritorhymes

assets/sass/main.scss

@@ -14,13 +14,13 @@ body {
   padding-bottom: .1em;
   border-radius: 2em;
 }
+img {
+  max-width: 100%;
+  height: auto;
+}
 .banner {
   text-align: center;
   font-weight: bold;
-  img {
-    max-width: 100%;
-    height: auto;
-  }
 }
 big {
   font-size: 400%;