fix(desktop): nur eine Instanz — zweiter Start holt das Fenster nach vorne

- tauri-plugin-single-instance als erstes Plugin registriert
- show_main_window-Helper ersetzt die dreifach duplizierte Fenster-Sequenz

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/desktop/src-tauri/Cargo.lock

@@ -4034,6 +4034,22 @@ dependencies = [
  "zbus",
 ]
 
+[[package]]
+name = "tauri-plugin-single-instance"
+version = "2.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "db817fe9295e19b7d8357e900af31edb93703dd9fb6de524b007b47b6afc63b0"
+dependencies = [
+ "serde",
+ "serde_json",
+ "tauri",
+ "thiserror 2.0.18",
+ "tokio",
+ "tracing",
+ "windows-sys 0.60.2",
+ "zbus",
+]
+
 [[package]]
 name = "tauri-plugin-store"
 version = "2.4.3"
@@ -4246,6 +4262,7 @@ dependencies = [
  "tauri-plugin-autostart",
  "tauri-plugin-notification",
  "tauri-plugin-opener",
+ "tauri-plugin-single-instance",
  "tauri-plugin-store",
  "tauri-plugin-updater",
  "tauri-plugin-window-state",