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/desktopApp/build.gradle.kts

@@ -9,9 +9,17 @@ plugins {
 val composeVersion = "1.11.1"
 val os = OperatingSystem.current()
 val arch = System.getProperty("os.arch") ?: "amd64"
-// CI passes -PappVersion=<version> (release workflow); local builds keep the
-// default. jpackage requires a plain semver (no leading "v").
-val appVersion = (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. jpackage requires a
+// plain semver (no leading "v").
+val appVersion =
+    (project.findProperty("appVersion") as? String)
+        ?: project
+            .file("../../VERSION")
+            .takeIf { it.exists() }
+            ?.readText()
+            ?.trim()
+        ?: "0.1.0"
 val desktopTarget =
     when {
         os.isMacOsX -> if (arch == "aarch64") "macos-arm64" else "macos-x64"