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/sources/pages/dashboard/dashboard.js

@@ -1,2 +1,10 @@
+customElements.define('dashboard-page', class extends HTMLElement {})
+
 export function init(main) {
+  main.querySelector('dashboard-page').innerHTML = `
+    <div class="dashboard">
+      <h2>Dashboard</h2>
+      <p>Page d'exemple chargée via le routeur Syn.</p>
+    </div>
+  `
 }