fix: preloader + update notification robustness

- Add full-screen CSS preloader to webapp (fades out when _initApp completes)
- Defer _checkWebappUpdate() to 6s after app init so it does not compete
  with startup API calls (fixes perceived slowness on first load)
- Switch update-check throttle from sessionStorage to localStorage (6h TTL);
  use release published_at instead of version string for comparison, so the
  banner correctly appears when a new release is published regardless of whether
  the tag is a semver or the rolling "latest" tag
- PHP _isLatestVersion(): return true (do not suppress error reports) when
  tag_name is non-semver (e.g. "latest") — was incorrectly blocking ALL reports
- Kiosk checkForUpdates(): show banner only when the release asset actually
  contains an APK for the component; handle non-semver tag by treating it
  as always-update (prevents silent no-op with rolling "latest" tag)
- Scale gateway checkForUpdates(): same non-semver fix; apkUrl now defaults
  to empty and bails out if no matching APK found in assets (prevents 404 install)

evershelf-scale-gateway/app/src/main/kotlin/it/dadaloop/evershelf/scalegate/MainActivity.kt

@@ -427,10 +427,10 @@ class MainActivity : AppCompatActivity(), BleScaleListener, ServerEventListener
                 val latestTag = json.optString("tag_name", "").ifEmpty { return@Thread }
                 val current   = try { packageManager.getPackageInfo(packageName, 0).versionName ?: "" } catch (_: Exception) { "" }
                 val norm = { v: String -> v.trimStart('v') }
-                if (norm(latestTag) == norm(current)) return@Thread  // already up to date
+                val isSemver = latestTag.trimStart('v').matches(Regex("\\d+\\.\\d+.*"))
 
                 // Find scale-gateway APK in release assets
-                var apkUrl = APK_DOWNLOAD_URL
+                var apkUrl = ""
                 val assets = json.optJSONArray("assets")
                 if (assets != null) {
                     for (i in 0 until assets.length()) {
@@ -442,7 +442,13 @@ class MainActivity : AppCompatActivity(), BleScaleListener, ServerEventListener
                         }
                     }
                 }
-                val msg = "⬆️ Scale Gateway $current → $latestTag"
+                // Only show banner if the release actually contains our APK
+                if (apkUrl.isEmpty()) return@Thread
+                // If semver tag matches current version → already up to date
+                if (isSemver && norm(latestTag) == norm(current)) return@Thread
+
+                val label = if (isSemver) "$current → $latestTag" else latestTag
+                val msg = "⬆️ Scale Gateway $label"
                 runOnUiThread { showNativeUpdateBanner(msg, apkUrl) }
             } catch (_: Exception) {}
         }.start()