fix: prevent duplicate widget windows on Windows/Linux (single instance)

Add tauri-plugin-single-instance (Windows/Linux only) registered as the
first plugin. A second launch — autostart, deep link, or re-open — now
focuses the running app instead of spawning another process with its own
floating widget. macOS enforces a single instance natively, which is why
the bug never appeared there. The deep-link feature forwards auth URLs
from a blocked second instance to the running app.

src-tauri/Cargo.lock

@@ -4958,6 +4958,7 @@ dependencies = [
  "tauri-plugin-opener",
  "tauri-plugin-process",
  "tauri-plugin-shell",
+ "tauri-plugin-single-instance",
  "tauri-plugin-store",
  "tauri-plugin-updater",
  "tokio",
@@ -5332,6 +5333,22 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tauri-plugin-single-instance"
+version = "2.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a33a5b7d78f0dec4406b003ea87c40bf928d801b6fd9323a556172c91d8712c1"
+dependencies = [
+ "serde",
+ "serde_json",
+ "tauri",
+ "tauri-plugin-deep-link",
+ "thiserror 2.0.18",
+ "tracing",
+ "windows-sys 0.60.2",
+ "zbus",
+]
+
 [[package]]
 name = "tauri-plugin-store"
 version = "2.4.2"

src-tauri/Cargo.toml

@@ -54,6 +54,11 @@ objc2-foundation = "0.3"
 [target.'cfg(any(target_os = "macos", windows, target_os = "linux"))'.dependencies]
 tauri-plugin-autostart = "2.5.1"
 
+# macOS enforces a single app instance natively; Windows/Linux need this plugin
+# so a second launch focuses the running app instead of spawning another widget.
+[target.'cfg(any(windows, target_os = "linux"))'.dependencies]
+tauri-plugin-single-instance = { version = "2", features = ["deep-link"] }
+
 [target.'cfg(target_os = "linux")'.dependencies]
 pipewire = "0.10.0"
 webkit2gtk = { version = "2.0.2", features = ["v2_8"] }

src-tauri/src/lib.rs

@@ -22,8 +22,41 @@ use tauri::{Emitter, Manager};
 use tauri_plugin_deep_link::DeepLinkExt;
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
+/// Bring the already-running instance to front when a second launch is blocked
+/// by the single-instance plugin (Windows/Linux only).
+#[cfg(any(windows, target_os = "linux"))]
+fn focus_existing_instance(app: &tauri::AppHandle) {
+    use tauri::Manager;
+
+    if let Some(win) = app.get_webview_window("settings") {
+        let _ = win.unminimize();
+        let _ = win.show();
+        let _ = win.set_focus();
+        return;
+    }
+
+    if let Some(win) = app.get_webview_window("widget") {
+        let _ = win.show();
+        let _ = win.set_focus();
+    }
+}
+
 pub fn run() {
-    tauri::Builder::default()
+    #[allow(unused_mut)]
+    let mut builder = tauri::Builder::default();
+
+    // The single-instance plugin must be registered first. macOS enforces one
+    // app instance natively (so the duplicate-widget bug never happens there);
+    // on Windows/Linux a second launch — autostart, deep link, or re-open —
+    // would otherwise start another process with its own floating widget.
+    #[cfg(any(windows, target_os = "linux"))]
+    {
+        builder = builder.plugin(tauri_plugin_single_instance::init(
+            |app, _argv, _cwd| focus_existing_instance(app),
+        ));
+    }
+
+    builder
         .plugin(tauri_plugin_deep_link::init())
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_opener::init())