Release version management: single VERSION file as source of truth

- VERSION at repo root (0.1.2); bump it to cut a release
- App: generated AppVersion.kt (config-cache-safe Gradle task with
  VERSION as declared input) shown in Settings; sent to the gateway
  via X-Iris-App-Version header on the SSE open
- Gateway: reports its own version in hello.ack server_caps.app_version
  (read from the repo-root VERSION via the plugin symlink); stores the
  app's version in the device registry caps (merge, not overwrite, so
  an old app reconnecting without the header doesn't wipe it)
- Settings: app + gateway version rows, mismatch hint, and a best-effort
  Gitea latest-release check (ReleaseCheck) with an 'update available' hint
- Release workflow: reads VERSION from the repo (no manual input), with
  a guard against an empty file
- Docs: frames.schema.json + 04-wire-protocol.md updated for app_version

app/androidApp/build.gradle.kts

@@ -16,9 +16,16 @@ android {
         // Play Store requires an incrementing versionCode per upload; CI can
         // pass -PappVersionCode=<n>. Local builds keep the default.
         versionCode = (project.findProperty("appVersionCode")?.toString()?.toIntOrNull()) ?: 1
-        // CI passes -PappVersion=<version> (release workflow); local builds
-        // keep the default.
-        versionName = (project.findProperty("appVersion") as? String) ?: "0.1.0"
+        // The repo-root VERSION file is the single source of truth (bump it
+        // to cut a release); CI can still override with -PappVersion.
+        versionName =
+            (project.findProperty("appVersion") as? String)
+                ?: project
+                    .file("../../VERSION")
+                    .takeIf { it.exists() }
+                    ?.readText()
+                    ?.trim()
+                ?: "0.1.0"
     }
 
     buildTypes {