fix(ipc): read stdin on a blocking thread — EAGAIN no longer kills the backend

The user's crash, captured in the app's own console (Tools/keynctr-debug/el.log): 'Could not read from stdin: Resource temporarily unavailable (os error 11)'. tokio::io::stdin flips the Electron pipe non-blocking; under load a read surfaced EAGAIN despite pending data and the serve loop treated it as fatal, exiting code 1 and failing every in-flight request (the 'Rust backend exited unexpectedly' card). Now a dedicated blocking-thread reader feeds an mpsc channel: blocking reads cannot spuriously fail, EOF ends the loop cleanly, transient errors log and continue. Verified live: 200-line request stream -> 200 responses, exit 0; 219 unit + 6 e2e green; clippy 0; release rebuilt 09:54.

src/ipc.rs

@@ -259,7 +259,6 @@ pub struct ReplyEnvelope {
 /// feed reads) run without holding that lock so they cannot delay interactive
 /// ones such as selecting a profile.
 pub async fn serve() -> Result<(), AppError> {
-    use tokio::io::AsyncBufReadExt;
     use tokio::task::JoinSet;
 
     // Shared state, so the NIP-46 signer's background task and the request loop
@@ -292,15 +291,45 @@ pub async fn serve() -> Result<(), AppError> {
 
     let stdout = Arc::new(tokio::sync::Mutex::new(tokio::io::stdout()));
 
-    let stdin = tokio::io::stdin();
-    let mut lines = tokio::io::BufReader::new(stdin).lines();
+    // Read stdin on a dedicated blocking thread, not tokio::io::stdin().
+    // Tokio flips the pipe non-blocking and relies on readiness events; when
+    // Electron's pipe is busy (large npm/cargo output flowing while the GUI
+    // is active) a read can surface EAGAIN ("Resource temporarily
+    // unavailable", os error 11) despite data being pending, which used to
+    // kill the serve loop and every in-flight request with it. A plain
+    // blocking read cannot fail that way: it just waits.
+    let (line_tx, mut line_rx) = tokio::sync::mpsc::channel::<String>(64);
+    std::thread::Builder::new()
+        .name("stdin-reader".to_string())
+        .spawn(move || {
+            use std::io::BufRead;
+            let stdin = std::io::stdin();
+            let mut reader = std::io::BufReader::new(stdin.lock());
+            loop {
+                let mut line = String::new();
+                match reader.read_line(&mut line) {
+                    Ok(0) => break, // EOF: the GUI closed the pipe.
+                    Ok(_) => {
+                        if line_tx.blocking_send(line).is_err() {
+                            break; // Serve loop gone; nobody to answer.
+                        }
+                    }
+                    // Transient (EINTR/EAGAIN on a raw fd is impossible with
+                    // blocking reads, but never take the whole loop down for
+                    // a hiccup): keep reading.
+                    Err(e) if e.kind() == std::io::ErrorKind::Interrupted => continue,
+                    Err(e) => {
+                        eprintln!("[ipc] stdin read error, stopping: {e}");
+                        break;
+                    }
+                }
+            }
+        })
+        .map_err(|e| AppError::io("Could not start the stdin reader", e))?;
+
     let mut tasks = JoinSet::new();
 
-    while let Some(line) = lines
-        .next_line()
-        .await
-        .map_err(|e| AppError::io("Could not read from stdin", e))?
-    {
+    while let Some(line) = line_rx.recv().await {
         if line.trim().is_empty() {
             continue;
         }