feat: panneau A propos elegant + retrait Editor POC

- Remplace Editor POC par A propos (sparkles) dans le menu header
- Modale About avec animation fade+scale, logo SVG, hero gradient
- Stats live: fichiers, vaults (API /health), tests (285)
- Stack techno: 12 badges (FastAPI, vanilla JS, Docker, etc.)
- Footer: date livraison, blague aleatoire (10 variantes humour)
- Touche d'humour: git push --force accidentels, CSS > backend
- Fermeture: bouton X, clic overlay, Escape
- initAboutModal dans sync.js + config.js export

frontend/js/sync.js

@@ -3,7 +3,7 @@ import { state } from './state.js';
 import { showToast } from './ui.js';
 import { api } from './auth.js';
 import { loadVaults, loadTags, refreshTagsForContext, refreshSidebarTreePreservingState } from './sidebar.js';
-import { loadRecentFiles } from './config.js';
+import { loadRecentFiles, initAboutModal } from './config.js';
 import { openFile } from './viewer.js';
 import { initOffline, syncFileIndexFromServer } from './offline.js';
 import { initThemes } from './themes.js';
@@ -554,6 +554,7 @@ export function init() {
   registerServiceWorker();
   initOffline();
   initThemes();
+  initAboutModal();
   // Sync file index after a short delay (let vaults load first)
   setTimeout(() => syncFileIndexFromServer(), 3000);
   initDashboardTabs();