fix(run): refuse a run with ETMUXMISSING when the host has no tmux

Starting a run on a machine without tmux surfaced `spawnSync tmux ENOENT`
as a 400 — blaming the request for a missing host package, naming no fix,
and reaching the user as a raw errno in the console pane.

The guard goes in spawnRun, which both spawn paths (POST /api/run and
POST /api/lanes/:id/start) already route through, so one check covers
them; the message carries the install command. Both routes map the code to
503: the request was fine, the dependency is not installed. The client
already renders `error.message` from the body verbatim, so no UI change.

Also: README's node advice said "stay on 20/22/24"; 1712 server + 488
client tests are green on v26.8.1, so only 25 is actually the bad one.

server/routes/run.js

@@ -260,6 +260,11 @@ router.post("/", (req, res) => {
     if (err.code === "ERUNLIVE") {
       return res.status(409).json({ error: { code: err.code, message: err.message } });
     }
+    // Nor is a host missing tmux: the request was fine, the dependency is not
+    // installed. 400 would tell the user to fix their input.
+    if (err.code === "ETMUXMISSING") {
+      return res.status(503).json({ error: { code: err.code, message: err.message } });
+    }
     if (err.code && err.code.startsWith("E")) {
       return res.status(400).json({ error: { code: err.code, message: err.message } });
     }