clean exemple/navigation to a minimal working example

- index.html: fix title and typo, add missing #error div
- config.json: replace project-specific keys with generic api_url
- core/menu.js: remove Voltalis mock data, leave commented usage pattern
- dashboard.js: move logic to init(), Custom Element is a plain shell
- dashboard.css: add real styles, remove placeholder comment
- manifest.json: drop unused version field
- config.yml: fix web/vendor comment → sources/vendor

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: GnomeZworc <nicolas.boufidjeline@g3e.fr>

exemple/navigation/config.yml

@@ -22,7 +22,7 @@ components:
     repo: https://git.g3e.fr/syonad/syn-side-nav
     ref: main
 
-# Vendor libraries — downloaded into web/vendor/ at build time (gitignored).
+# Vendor libraries — downloaded into sources/vendor/ at build time (gitignored).
 # Components import them via relative paths, never via components.json.
 # path: restrict download to a subdirectory of the repo (optional).
 # dest: destination under web/ (default: vendor/<name>).