fix(installer): surface manage.sh stderr when Pulse install fails

[stevetr68]

Problem

When Pulse fails to install, installPulse() reports a generic message and discards the
real cause. It spawns manage.sh install with stderr piped, but never reads the pipe:

const installOk = await new Promise<boolean>((resolve) => {
  const child = spawn("bash", [manageScript, "install"], {
    cwd: pulseDir,
    stdio: ["ignore", "pipe", "pipe"],   // stderr piped...
  });
  const timer = setTimeout(() => { child.kill(); resolve(false); }, 30000);
  child.on("close", (code) => { clearTimeout(timer); resolve(code === 0); });  // ...but never read
  child.on("error", () => { clearTimeout(timer); resolve(false); });
});

if (!installOk) {
  await emit({ event: "message", content: "Pulse install command failed. Voice notifications will not be available." });
  return false;
}

So a real failure inside manage.sh (missing ~/Library/LaunchAgents, a sed/launchctl
error, etc.) is swallowed — the installer prints "install command failed" with no detail
and the user is left guessing.

Fix

Accumulate the already-piped stderr and include its tail in the failure message. No new
process behavior, no extra spawn — just stop throwing away output we're already capturing.

let installStderr = "";
// ...
child.stderr?.on("data", (chunk) => { installStderr += chunk.toString(); });
// ...
if (!installOk) {
  const tail = installStderr.trim().split("\n").slice(-20).join("\n");
  await emit({ event: "message", content: "Pulse install command failed. Voice notifications will not be available." + (tail ? "\nmanage.sh stderr (last lines):\n" + tail : "") });
  return false;
}

Pairs with #1310 — that fixes one common cause of the silent failure; this makes any
remaining cause visible instead of opaque.