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

index.html

@@ -11,7 +11,7 @@
     <title>EverShelf</title>
     <link rel="manifest" href="manifest.json">
     <link rel="icon" type="image/png" href="assets/img/logo/logo_icon.png">
-    <link rel="stylesheet" href="assets/css/style.css?v=20260505b">
+    <link rel="stylesheet" href="assets/css/style.css?v=20260506a">
     <!-- QuaggaJS for barcode scanning -->
     <script src="https://cdn.jsdelivr.net/npm/@ericblade/quagga2@1.8.4/dist/quagga.min.js"></script>
     <!-- @xenova/transformers: ES-module bootstrap that exposes a lazy category-classifier as window._categoryPipelinePromise -->
@@ -67,7 +67,7 @@
         <!-- Title — left-aligned; grows to fill space -->
         <div class="header-title-wrap">
             <h1 class="header-title" onclick="showPage('dashboard')">
-                <img src="assets/img/logo/logo_icon.png" alt="" class="header-logo-icon" aria-hidden="true" /><span data-i18n="nav.title">EverShelf</span><span class="header-version">v1.7.2</span>
+                <img src="assets/img/logo/logo_icon.png" alt="" class="header-logo-icon" aria-hidden="true" /><span data-i18n="nav.title">EverShelf</span><span class="header-version">v1.7.3</span>
             </h1>
             <!-- Update badge — shown alongside title, never replaces it -->
             <span class="header-update-badge" id="header-update-badge" style="display:none"></span>