pdf: ignore safe area insets in fullscreen mode (#484)

Add padding: 0 !important to .main.fullscreen so that
safe area insets applied via inline styles are overridden,
making the PDF viewer truly fullscreen edge-to-edge.

Also add .build directories to .prettierignore to fix
pre-commit hook failures from Swift build artifacts.

examples/pdf-server/src/mcp-app.css

@@ -325,6 +325,7 @@ body {
   overflow: hidden; /* No scrolling on main - only canvas-container scrolls */
   border-radius: 0;
   border: none;
+  padding: 0 !important; /* Ignore safe area insets in fullscreen */
 }
 
 .main.fullscreen .viewer {