Add single-instance tauri plugin (#261)

Co-authored-by: lukas <lukas.walter@aceart.de>

src-tauri/Cargo.toml

@@ -18,6 +18,7 @@ tauri-build = { version = "1.5.2", features = [] }
 serde_json = "1.0.109"
 serde = { version = "1.0.193", features = ["derive"] }
 tauri = { version = "1.6.7", features = ["api-all", "devtools", "system-tray", "updater"] }
+tauri-plugin-single-instance = { git = "https://github.com/tauri-apps/plugins-workspace", branch = "v1" }
 
 [features]
 # by default Tauri runs in production mode