Fix broken braindecode logo URL

The previous /_images/braindecode.svg path returns 404 on braindecode.org;
the logo now lives under /_static/braindecode_long.svg.

frontend/src/components/Logo/Logo.js

@@ -68,7 +68,7 @@ const Logo = ({ height = "40px" }) => {
       >
         <Box
           component="img"
-          src="https://braindecode.org/stable/_images/braindecode.svg"
+          src="https://braindecode.org/stable/_static/braindecode_long.svg"
           alt="Braindecode"
           sx={{
             height: "100%",