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

.github/workflows/build-scale-gateway.yml

@@ -4,6 +4,9 @@ on:
   push:
     branches:
       - main
+      - develop
+    paths:
+      - 'evershelf-scale-gateway/**'
   workflow_dispatch:
 
 permissions: