Fix app hanging on sidecar startup

Two issues caused the app to freeze on "Starting sidecar...":

1. wait_for_ready() used a blocking BufReader::lines() iterator
   with a timeout check between lines. If the sidecar produced no
   stdout output (crashed, missing binary, or slow model loading),
   the read blocked forever. Now uses a background thread with
   mpsc::recv_timeout() for a real 120s deadline.

2. start_sidecar was a synchronous Tauri command that blocked the
   main thread during the entire sidecar startup (up to 120s).
   Now async via tokio::spawn_blocking, keeping the UI responsive.

Also logs all sidecar stdout lines to stderr with [sidecar-stdout]
prefix for debugging.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -29,9 +29,9 @@ pub fn run() {
         .plugin(tauri_plugin_shell::init())
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_process::init())
-        .manage(sidecar::ManagedSidecar(Mutex::new(
+        .manage(sidecar::ManagedSidecar(std::sync::Arc::new(Mutex::new(
             sidecar::SidecarManager::new(),
-        )))
+        ))))
         .setup(|app| {
             let resource_dir = app
                 .path()

src-tauri/src/sidecar/mod.rs

@@ -587,23 +587,44 @@ impl SidecarManager {
     }
 
     fn wait_for_ready(stdout: std::process::ChildStdout) -> Result<u16, String> {
-        let reader = std::io::BufReader::new(stdout);
-        let timeout = std::time::Duration::from_secs(120);
-        let start = std::time::Instant::now();
+        use std::sync::mpsc;
 
-        for line in reader.lines() {
-            if start.elapsed() > timeout {
-                return Err("Timed out waiting for sidecar ready event".into());
-            }
-            let line = line.map_err(|e| format!("IO error reading stdout: {e}"))?;
-            if let Ok(evt) = serde_json::from_str::<ReadyEvent>(&line) {
-                if evt.event == "ready" {
-                    return Ok(evt.port);
+        let timeout = std::time::Duration::from_secs(120);
+
+        // Read stdout in a background thread so we can enforce a real timeout.
+        // BufReader::lines() blocks indefinitely if no data arrives.
+        let (tx, rx) = mpsc::channel();
+
+        std::thread::spawn(move || {
+            let reader = std::io::BufReader::new(stdout);
+            for line in reader.lines() {
+                match line {
+                    Ok(line) => {
+                        eprintln!("[sidecar-stdout] {}", line);
+                        if let Ok(evt) = serde_json::from_str::<ReadyEvent>(&line) {
+                            if evt.event == "ready" {
+                                let _ = tx.send(Ok(evt.port));
+                                return;
+                            }
+                        }
+                    }
+                    Err(e) => {
+                        let _ = tx.send(Err(format!("IO error reading stdout: {e}")));
+                        return;
+                    }
                 }
             }
-            // Ignore other lines (e.g. log output)
-        }
-        Err("Sidecar process exited before sending ready event".into())
+            let _ = tx.send(Err(
+                "Sidecar process exited before sending ready event".into(),
+            ));
+        });
+
+        rx.recv_timeout(timeout).unwrap_or_else(|_| {
+            Err(format!(
+                "Timed out after {}s waiting for sidecar ready event",
+                timeout.as_secs()
+            ))
+        })
     }
 }
 
@@ -612,7 +633,8 @@ impl SidecarManager {
 // ---------------------------------------------------------------------------
 
 /// Wrapper so we can store `SidecarManager` in Tauri's managed state.
-pub struct ManagedSidecar(pub Mutex<SidecarManager>);
+/// Uses Arc so it can be cloned into background threads for async commands.
+pub struct ManagedSidecar(pub std::sync::Arc<Mutex<SidecarManager>>);
 
 #[tauri::command]
 pub fn get_sidecar_port(state: tauri::State<'_, ManagedSidecar>) -> Result<Option<u16>, String> {
@@ -628,12 +650,16 @@ pub fn get_sidecar_port(state: tauri::State<'_, ManagedSidecar>) -> Result<Optio
 }
 
 #[tauri::command]
-pub fn start_sidecar(state: tauri::State<'_, ManagedSidecar>) -> Result<u16, String> {
-    let mut mgr = state
-        .0
-        .lock()
-        .map_err(|e| format!("Lock error: {e}"))?;
-    mgr.ensure_running()
+pub async fn start_sidecar(state: tauri::State<'_, ManagedSidecar>) -> Result<u16, String> {
+    let mgr = state.0.clone();
+    // Run blocking sidecar launch in a background thread so it doesn't
+    // freeze the Tauri UI while waiting for the ready event (up to 120s).
+    tokio::task::spawn_blocking(move || {
+        let mut mgr = mgr.lock().map_err(|e| format!("Lock error: {e}"))?;
+        mgr.ensure_running()
+    })
+    .await
+    .map_err(|e| format!("Task join error: {e}"))?
 }
 
 #[tauri::command]