screenshare: surface pixelpass stderr on host startup failure

When a pixelpass host dies before emitting its ticket (e.g. a fresh Ubuntu
box missing the GStreamer capture stack), we discarded its stderr — which is
exactly where pixelpass prints the failing element plus an actionable
"Install hint: sudo apt install ..." line. The user only saw a dead-end
"pixelpass host exited before emitting a ticket".

Capture stderr instead of nulling it, and on the early-exit path fold a short
tail into our error via the pure `pixelpass_failure_detail` (drops the
decorative banner, keeps the error + hint, caps at the last 12 lines). On the
success path stderr is drained in the background so its pipe can't stall the
host. +3 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/screenshare/mod.rs

@@ -332,7 +332,13 @@ pub async fn spawn_host(
         .args(host_args(audio_app))
         .stdin(Stdio::null())
         .stdout(Stdio::piped())
-        .stderr(Stdio::null())
+        // Capture stderr (not null): pixelpass prints its startup precondition
+        // failures there — a missing GStreamer plugin / `pactl`, each with an
+        // actionable "Install hint: sudo apt install ..." line. If the host dies
+        // before its ticket we fold that tail into our error so the user sees
+        // *what to install* instead of a dead-end "exited before a ticket". On
+        // the success path we drain it in the background so the pipe can't fill.
+        .stderr(Stdio::piped())
         .kill_on_drop(true)
         .spawn()?;
 
@@ -340,6 +346,7 @@ pub async fn spawn_host(
         .stdout
         .take()
         .ok_or_else(|| std::io::Error::other("pixelpass host stdout missing"))?;
+    let stderr = child.stderr.take();
     let mut lines = BufReader::new(stdout).lines();
 
     let ticket = match read_until(&mut lines, |e| match e {
@@ -351,9 +358,10 @@ pub async fn spawn_host(
         Ok(Some(t)) => t,
         Ok(None) => {
             let _ = child.kill().await;
-            return Err(std::io::Error::other(
-                "pixelpass host exited before emitting a ticket",
-            ));
+            let detail = read_stderr_tail(stderr).await;
+            return Err(std::io::Error::other(format!(
+                "pixelpass host exited before emitting a ticket{detail}"
+            )));
         }
         Err(e) => {
             let _ = child.kill().await;
@@ -361,10 +369,65 @@ pub async fn spawn_host(
         }
     };
 
+    if let Some(stderr) = stderr {
+        drain_stderr_in_background(stderr);
+    }
     drain_in_background(lines, "host", notices);
     Ok((child, ticket))
 }
 
+/// Read a killed pixelpass child's stderr to EOF and reduce it to a short,
+/// user-facing diagnostic tail via [`pixelpass_failure_detail`]. Bounded: the
+/// caller kills the child first, so the pipe EOFs promptly. Returns an empty
+/// string when stderr was already taken or carried nothing useful.
+async fn read_stderr_tail(stderr: Option<tokio::process::ChildStderr>) -> String {
+    use tokio::io::AsyncReadExt;
+    let Some(mut stderr) = stderr else {
+        return String::new();
+    };
+    let mut buf = Vec::new();
+    let _ = stderr.read_to_end(&mut buf).await;
+    pixelpass_failure_detail(&String::from_utf8_lossy(&buf))
+}
+
+/// Discard a running pixelpass child's stderr in the background so its pipe
+/// can't fill and stall the host (mirrors [`drain_in_background`] for stdout).
+fn drain_stderr_in_background(mut stderr: tokio::process::ChildStderr) {
+    use tokio::io::AsyncReadExt;
+    tokio::spawn(async move {
+        let mut buf = [0u8; 4096];
+        while let Ok(n) = stderr.read(&mut buf).await {
+            if n == 0 {
+                break;
+            }
+        }
+    });
+}
+
+/// Extract a human-useful tail from a failed pixelpass child's stderr to append
+/// to our error. pixelpass writes actionable startup errors there (a missing
+/// GStreamer element / `pactl` plus an `Install hint: sudo apt install ...`
+/// line), which is exactly what a freshly-installed host needs to see. The
+/// decorative host banner (box-drawing) is dropped — it only prints on the
+/// success path, but we filter it defensively. Pure: no I/O. Returns an empty
+/// string when there's nothing worth surfacing (so callers can append blindly).
+pub fn pixelpass_failure_detail(stderr: &str) -> String {
+    let useful: Vec<&str> = stderr
+        .lines()
+        .map(str::trim_end)
+        .filter(|l| !l.trim().is_empty())
+        .filter(|l| !l.trim_start().starts_with(['│', '┌', '└', '├']))
+        .collect();
+    if useful.is_empty() {
+        return String::new();
+    }
+    // The anyhow error and its install hint are the *last* lines printed, so
+    // keep the tail rather than the head.
+    const MAX_LINES: usize = 12;
+    let start = useful.len().saturating_sub(MAX_LINES);
+    format!("\n\npixelpass reported:\n{}", useful[start..].join("\n"))
+}
+
 /// Spawn a pixelpass viewer for `ticket`, wait for it to connect, and open the
 /// stream in a local player (mpv, falling back to vlc). Returns the live viewer
 /// child so the caller can kill it on room-leave; it also self-exits when the
@@ -633,6 +696,48 @@ mod tests {
         assert_eq!(parse_audio_apps(stdout.as_bytes()), vec!["mpv".to_string()]);
     }
 
+    #[test]
+    fn failure_detail_surfaces_install_hint_and_drops_banner() {
+        // The real shape of a fresh-host failure: anyhow error + install hint on
+        // stderr. We must keep those (so the user knows what to apt install) and
+        // drop the decorative banner box-drawing lines.
+        let stderr = "\
+┌─ PixelPass · host ─────────────────────────────────────────
+│  display server : Wayland
+└────────────────────────────────────────────────────────────
+Error: GStreamer element `vah264enc` not available.
+Install hint: sudo apt install gstreamer1.0-plugins-bad
+";
+        let detail = pixelpass_failure_detail(stderr);
+        assert!(detail.starts_with("\n\npixelpass reported:\n"));
+        assert!(detail.contains("vah264enc` not available"));
+        assert!(detail.contains("sudo apt install gstreamer1.0-plugins-bad"));
+        assert!(!detail.contains('│'), "banner box-drawing must be dropped");
+        assert!(!detail.contains('┌'));
+    }
+
+    #[test]
+    fn failure_detail_empty_when_nothing_useful() {
+        // Blank / banner-only stderr yields an empty string so the caller can
+        // append it to the base message unconditionally without trailing noise.
+        assert_eq!(pixelpass_failure_detail(""), "");
+        assert_eq!(pixelpass_failure_detail("   \n  \n"), "");
+        assert_eq!(
+            pixelpass_failure_detail("│  display server : Wayland\n│  capture : x\n"),
+            ""
+        );
+    }
+
+    #[test]
+    fn failure_detail_keeps_only_the_tail() {
+        // A long stderr is truncated to its last lines (where the real error
+        // and hint live), not its head.
+        let body: String = (0..30).map(|i| format!("line {i}\n")).collect();
+        let detail = pixelpass_failure_detail(&body);
+        assert!(detail.contains("line 29"));
+        assert!(!detail.contains("line 0\n"));
+    }
+
     #[test]
     fn help_probe_detects_strict_audio_flag() {
         // A new pixelpass advertises the flag; an old one doesn't. The probe must