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

evershelf-kiosk/app/build.gradle.kts

@@ -11,8 +11,8 @@ android {
         applicationId = "it.dadaloop.evershelf.kiosk"
         minSdk = 24
         targetSdk = 34
-        versionCode = 4
-        versionName = "1.3.0"
+        versionCode = 5
+        versionName = "1.4.0"
     }
 
     signingConfigs {