chore: bump versions + update CHANGELOG/README for v1.6.0

Webapp:    v1.5.0 → v1.6.0
Kiosk:     v1.3.0 → v1.4.0 (versionCode 4→5)
Scale GW:  v2.0.0 → v2.1.0 (versionCode 6→7)

CI: build-scale-gateway.yml now also triggers on develop branch
    (was main-only, causing APK builds to not run on feature branches)

CHANGELOG: added [1.6.0] entry covering PackageInstaller OTA fixes,
  dashboard skeleton, update banners, cooking mode z-index, XOR token
README: updated 'Recent Updates' section with 1.6.0 highlights

index.html

@@ -61,7 +61,7 @@
 <!-- Top Header -->
 <header class="app-header">
     <div class="header-content">
-        <h1 class="header-title" onclick="showPage('dashboard')"><span data-i18n="nav.title">🏠 EverShelf</span><span class="header-version">v1.5.0</span></h1>
+        <h1 class="header-title" onclick="showPage('dashboard')"><span data-i18n="nav.title">🏠 EverShelf</span><span class="header-version">v1.6.0</span></h1>
         <div class="header-actions">
             <span id="scale-status-indicator" class="scale-status-indicator scale-status-disconnected" style="display:none" data-i18n-title="scale.status_disconnected" title="⚖️ Bilancia">⚖️</span>
             <button class="header-scan-btn header-gemini-btn" onclick="showPage('chat')" title="Chat con Gemini" data-i18n-title="chat.title">