fix: center header title + bump to v1.7.3 for update detection

- CSS: header-title-wrap uses position:absolute with left:0/right:0
  + justify-content:center so title is truly centered regardless of
  button count on each side (kiosk has 2 left / 3 right)
- header-actions gets margin-left:auto to push right; both sides z-index:1
- Bump version 1.7.2 → 1.7.3 so already-open kiosk tabs will see
  the update badge once the new version is deployed to the server
- CSS cache bust: ?v=20260506a

manifest.json

@@ -2,7 +2,7 @@
     "name": "EverShelf",
     "short_name": "EverShelf",
     "description": "Gestione completa della dispensa di casa con scansione barcode",
-    "version": "1.7.2",
+    "version": "1.7.3",
     "start_url": "/evershelf/",
     "display": "standalone",
     "background_color": "#f0f4e8",