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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/lib/motion.ts

@@ -5,7 +5,7 @@
  * across the app. Mirrors the `--ease-*` / `--duration-*` timing tokens defined
  * in `index.css`, so CSS transitions and JS animations stay in sync.
  *
- * This file is duplicated verbatim in `fluksio` and `website` (the shadcn model,
+ * This file is duplicated verbatim in `app` and `index` (the shadcn model,
  * see ../../DESIGN.md / the root DESIGN-GUIDELINES.md). Keep the two copies
  * identical; `make design-check` at the workspace root verifies it.
  *