style: add max-width to main pane for better readability

- Set max-width of 900px for the main content pane
- Center content with margin: 0 auto
- Add horizontal padding for breathing room
- Improves reading experience by preventing overly wide lines

src/index.css

@@ -461,6 +461,9 @@ body {
 .pane.main {
   overflow-y: auto;
   height: 100%;
+  max-width: 900px;
+  margin: 0 auto;
+  padding: 0 2rem;
 }
 
 .pane.highlights {