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/components/Footer.module.css

@@ -1,8 +1,6 @@
 .footer {
-  position: absolute;
-  right: 0;
-  bottom: 1rem;
-  left: 0;
+  flex-shrink: 0;
+  padding: 0 0 1rem;
   height: 20px;
   color: rgba(255, 255, 255, 0.5);
   font-size: 0.8rem;