fix(notes): audio overflow + volume 50%, grille 3col, bouton laptop, sidebar BDD

- Audio : minWidth:0 + onLoadedMetadata volume=0.5 (plus de débordement)
- Grille : repeat(3,1fr) sur laptop, 1fr sur mobile (était auto-fill)
- Header laptop : bouton "Nouvelle note" (fa-plus + accent) visible lg:flex
- SideNav : DbStatusBar en bas — LED verte/rouge + taille BDD, polling 30s
- docs/plan.md : Phase 4c documentée

v0.5.7

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "homehub-frontend",
   "private": true,
-  "version": "0.5.6",
+  "version": "0.5.7",
   "type": "module",
   "scripts": {
     "dev": "vite",