Fix sidebar height; scroll the button list when it overflows

README.md

@@ -3,7 +3,9 @@
 A single Python app that is both the frontend and the backend for a dashboard of
 internal services. The sidebar has one button per service; clicking it loads the
 service in an iframe **through the dashboard's built-in reverse proxy**, so the
-upstream services never need to be public.
+upstream services never need to be public. The sidebar is one viewport tall -
+once the buttons overflow it, its own list scrolls and the rest of the page
+stays put.
 
 ```
 browser ──► uwudash (host:port) ──► /proxy/<id>/<path> ──► upstream origin