Installable app + one-click update from forgejo

- scripts/update-app.sh: detached updater (setsid, survives app exit):
  git pull atitlan master -> gradle createDistributable -> only then
  kill app+engine and relaunch the fresh binary. Pull or build failure
  leaves the running app alone. Log: ~/.config/shonar-desktop/logs/update.log.
- Settings → About & updates: 'Install update' button appears when the
  check finds a newer remote commit; hands off to the script and tells
  the user the app will relaunch itself.
- Desktop entry (~/.local/share/applications/shonar-desktop.desktop,
  outside the repo): launches the packaged distributable via
  launch-desktop.sh; SHONAR_REPO fallback already resolves the tree,
  so the packaged app spawns the engine and sees the updater script.

Gate: :app:compileKotlin + :app:test green; desktop-file-validate clean;
launcher verified (app window from the menu path); updater fail-safe
verified (dirty-tree pull refused, app survived).

app/src/main/kotlin/com/shonar/desktop/DesktopState.kt

@@ -224,6 +224,42 @@ class DesktopState(private val appDir: File = defaultAppDir()) {
         }
     }
 
+    /** True when scripts/update-app.sh exists next to the repo the engine
+     *  was spawned from (dev-tree installs only; the update pulls from
+     *  forgejo, rebuilds the distributable, and relaunches). */
+    val updateScript: File?
+        get() = repoRoot()?.let { File(it, "scripts/update-app.sh") }
+            ?.takeIf { it.canExecute() }
+
+    /** Hand off to the detached updater (git pull + rebuild + relaunch).
+     *  The script runs via setsid so it outlives this process when the
+     *  updater kills the app after a good build. */
+    fun installUpdate() {
+        val script = updateScript
+        if (script == null) {
+            _updateStatus.value = UpdateStatus("result",
+                "No updater script in this install — update manually " +
+                "(git pull + rebuild).", url = REPO_WEB, stale = true)
+            return
+        }
+        _updateStatus.value = UpdateStatus("result",
+            "Updating: pulling from forgejo and rebuilding — the app will " +
+            "relaunch itself when it's done. Log: " +
+            "${appDir.absolutePath}/logs/update.log")
+        scope.launch {
+            runCatching {
+                ProcessBuilder("setsid", "nohup", script.absolutePath)
+                    .directory(script.parentFile.parentFile)
+                    .redirectOutput(ProcessBuilder.Redirect.DISCARD)
+                    .start()
+            }.onFailure {
+                _updateStatus.value = UpdateStatus("result",
+                    "Could not start the updater: ${it.message}",
+                    url = REPO_WEB, stale = true)
+            }
+        }
+    }
+
     private val _detail = MutableStateFlow<DetailUi?>(null)
     val detail: StateFlow<DetailUi?> = _detail.asStateFlow()
 

app/src/main/kotlin/com/shonar/desktop/Screens.kt

@@ -1451,6 +1451,11 @@ fun SettingsScreen(state: DesktopState) {
                     Text("Open repo")
                 }
             }
+            if (us.stale && state.updateScript != null) {
+                ShonarButton({ state.installUpdate() }) {
+                    Text("Install update")
+                }
+            }
         }
         val us = state.updateStatus.collectAsState().value
         if (us.message.isNotEmpty()) {