web: Indent <pre> with padding-left (no spaces)

Fixes #34

assets/css/main.css

@@ -15,4 +15,8 @@ body {
     transition: visibility 175ms, opacity 175ms;
     width: 100%;
     z-index: 999;               
+}
+
+pre {
+    padding-left: 4ex; /* approx 4 monospaced spaces */
 }