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()