Rename the workspace layout: fluksio -> app, website -> index

The repo moves to git.stroblme.de:Fluksio/app.git and its sibling to
Fluksio/index.git, so every cross-repo path reference moves with it. The
architecture sources the roadmap cites now live in the docs repo under
architecture/, and the brand marks in the index repo's src/assets.

Also replaces the placeholder README with one that reflects the actual layout.

frontend/src/components/Common/Footer.tsx

@@ -3,7 +3,7 @@ import { SiCodeberg } from "react-icons/si"
 const socialLinks = [
   {
     icon: SiCodeberg,
-    href: "https://codeberg.org/fluksio",
+    href: "https://git.stroblme.de/Fluksio",
     label: "Codeberg",
   },
 ]