Prevent duplicate desktop app instances

Notification clicks and repeated launches can enter the native app through OS-level activation paths, so the desktop runtime now registers Tauri's single-instance guard before sidecars or other plugins start. A second launch restores and focuses the existing main window instead of creating another app process.

Constraint: macOS and Windows launch paths must reuse the running desktop instance.
Rejected: Handle only notification click callbacks | does not cover Dock, shortcut, installer, or Windows relaunch paths.
Confidence: high
Scope-risk: narrow
Directive: Keep the single-instance plugin before sidecar startup so duplicate launches cannot create extra runtime processes.
Tested: SKIP_INSTALL=1 ./desktop/scripts/build-macos-arm64.sh
Tested: open -n built app twice; desktop_count stayed 1 with pid 69821
Tested: bun run check:native
Tested: cargo test --manifest-path desktop/src-tauri/Cargo.toml
Tested: cd desktop && bun run test -- desktopNotifications.test.ts
Tested: cd desktop && bun run lint

desktop/src-tauri/Cargo.lock

@@ -485,6 +485,7 @@ dependencies = [
  "tauri-plugin-notification",
  "tauri-plugin-process",
  "tauri-plugin-shell",
+ "tauri-plugin-single-instance",
  "tauri-plugin-updater",
  "tauri-runtime",
  "tauri-runtime-wry",
@@ -4386,6 +4387,21 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tauri-plugin-single-instance"
+version = "2.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c8f29386f5e9fdc699182388a33ee80a56de436d91b67459e86afef426282af"
+dependencies = [
+ "serde",
+ "serde_json",
+ "tauri",
+ "thiserror 2.0.18",
+ "tracing",
+ "windows-sys 0.60.2",
+ "zbus",
+]
+
 [[package]]
 name = "tauri-plugin-updater"
 version = "2.10.1"

desktop/src-tauri/Cargo.toml

@@ -23,3 +23,4 @@ serde_json = "1"
 anyhow = "1.0.102"
 portable-pty = "0.9.0"
 tauri-plugin-notification = "2"
+tauri-plugin-single-instance = "2"

desktop/src-tauri/src/lib.rs

@@ -1487,6 +1487,10 @@ mod tests {
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     let builder = tauri::Builder::default()
+        // Keep this first so duplicate launches are stopped before sidecars start.
+        .plugin(tauri_plugin_single_instance::init(|app, _args, _cwd| {
+            show_main_window(app);
+        }))
         .manage(ServerState::default())
         .manage(AdapterState::default())
         .manage(TerminalState::default())