fix(ui): header title always centered, actions to right, real-time update detection

CSS header fixes:
- .header-content: justify-content:flex-end so .header-actions (camera, Gemini)
  naturally stays at the right edge as a flex child
- .header-title: removed overflow:hidden and text-overflow:ellipsis that were
  clipping the version number; title stays absolutely centered
- Cleaned up unused max-width:none and margin:0 from previous broken attempt

Real-time webapp update detection:
- Added module-level _loadedVersion captured at page load (version in HTML header)
- _checkWebappUpdate() now has two checks:
  1. webapp_version from server vs _loadedVersion: if different, the server was
     updated since this page was loaded → show '🔄 Nuova versione disponibile' banner
  2. GitHub latest release vs _loadedVersion (existing behaviour)
  Different banner messages: deploy-changed shows simple reload prompt;
  release-newer shows version + changelog link (same as before)
- TTL reduced from 6h to 5 min so updates are detected quickly
- _checkWebappUpdate() now also fires on visibilitychange so the user sees
  the banner as soon as they return to the tab after a deploy

assets/css/style.css

@@ -108,10 +108,8 @@ body {
     position: relative;
     display: flex;
     align-items: center;
-    justify-content: center;
+    justify-content: flex-end;
     width: 100%;
-    max-width: none;
-    margin: 0;
 }
 
 .header-title {
@@ -128,8 +126,6 @@ body {
     pointer-events: auto;
     /* prevent title overlapping action buttons on small screens */
     max-width: calc(100% - 200px);
-    overflow: hidden;
-    text-overflow: ellipsis;
 }
 
 .header-version {