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

assets/css/style.css

@@ -123,6 +123,7 @@ body {
     display: flex;
     align-items: center;
     width: 100%;
+    position: relative;
     gap: 8px;
 }
 
@@ -132,17 +133,25 @@ body {
     display: flex;
     align-items: center;
     gap: 6px;
+    position: relative;
+    z-index: 1;
 }
 
-/* Title wrap — fills available space, left-aligned */
+/* Title wrap — absolutely centered in the full header width */
 .header-title-wrap {
-    flex: 1 1 0;
+    position: absolute;
+    left: 0;
+    right: 0;
     display: flex;
+    justify-content: center;
     align-items: center;
     gap: 8px;
-    min-width: 0;
+    pointer-events: none;
     overflow: hidden;
 }
+.header-title-wrap > * {
+    pointer-events: auto;
+}
 
 .header-title {
     font-size: 1.3rem;
@@ -218,9 +227,12 @@ body {
 /* Actions — fixed-width, no stretching */
 .header-actions {
     flex: 0 0 auto;
+    margin-left: auto;
     display: flex;
     gap: 8px;
     align-items: center;
+    position: relative;
+    z-index: 1;
 }
 
 /* ── Base header button — all action icons use this ─────────────────── */