Desktop: first-run setup wizard + FlyWithLua/Web-API/Lua-status guidance

Adds the four onboarding pieces that were missing:
- flywithlua_present Tauri command + wizard step that checks the plugin and
  links the FlyWithLua NG+ download when it's absent.
- Wizard step explaining how to enable X-Plane's Web/REST API (Settings>Network).
- FlyWithLua-Sync status row in the live diagnostics, from /api/health.lua
  ('N Skripte aktiv' / 'FlyWithLua fehlt' / 'kein X-Plane').
- 4-step guided wizard (X-Plane folder → FlyWithLua → Web-API → install+start)
  that auto-opens on first launch and is reachable via the header Einrichten
  button; the final step hands off to the normal server start (auto-installs Lua).

Verified the wizard DOM flow + the dLua status against a live bridge.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

desktop/src-tauri/src/lib.rs

@@ -97,6 +97,19 @@ fn server_running(state: State<ServerState>) -> bool {
     state.child.lock().unwrap().is_some()
 }
 
+// Is FlyWithLua NG+ installed in this X-Plane? It's the prerequisite for the
+// FMS/terrain sync — the bridge auto-installs OUR scripts into its Scripts
+// folder, but only if the plugin itself is present. Checked by the setup wizard.
+#[tauri::command]
+fn flywithlua_present(path: String) -> bool {
+    !path.is_empty()
+        && PathBuf::from(&path)
+            .join("Resources")
+            .join("plugins")
+            .join("FlyWithLua")
+            .is_dir()
+}
+
 #[tauri::command]
 async fn start_server(
     app: tauri::AppHandle,
@@ -203,6 +216,7 @@ pub fn run() {
             suggest_port,
             default_xplane_path,
             valid_xplane_path,
+            flywithlua_present,
             server_running,
             start_server,
             stop_server