Keep statistics charts clear of the footer

Use a full-height flex layout and normal footer flow so long chart content scrolls without overlapping the footer. Keep the page gradient fixed while scrolling.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/App.module.css

@@ -1,4 +1,5 @@
 .app {
+  flex: 1 0 auto;
   max-width: 1220px;
   margin: 1rem auto;
 }