fix: dedupe concurrent gateway restarts + surface restart outcome in onboarding UI

Follow-ups to the salvaged Telegram QR onboarding auto-restart:

- _spawn_gateway_restart() reuses a live in-flight 'hermes gateway restart'
  child instead of spawning a second racing one (stale cached frontend +
  new backend both requesting a restart, or restart-button double-click).
  Both /api/gateway/restart and the onboarding apply path go through it.
- ChannelsPage polls /api/actions/gateway-restart/status after a
  server-initiated restart and surfaces a non-zero exit (e.g. systemd
  linger missing) via the manual-restart banner, since restart_started
  only means the child spawned.
- Test for the reuse path + _ACTION_PROCS isolation in existing tests.

hermes_cli/web_server.py

@@ -1372,11 +1372,28 @@ def _tail_lines(path: Path, n: int) -> List[str]:
     return lines[-n:] if n > 0 else lines
 
 
+def _spawn_gateway_restart() -> Tuple[subprocess.Popen, bool]:
+    """Spawn ``hermes gateway restart``, reusing an in-flight restart.
+
+    Multiple dashboard paths can request a restart in quick succession
+    (restart button double-click, or a stale cached frontend firing its own
+    restart after the server already auto-restarted post-onboarding). Two
+    concurrent ``hermes gateway restart`` children race each other on the
+    manual kill-and-start path, so reuse the live one instead.
+
+    Returns ``(proc, reused)``.
+    """
+    existing = _ACTION_PROCS.get("gateway-restart")
+    if existing is not None and existing.poll() is None:
+        return existing, True
+    return _spawn_hermes_action(["gateway", "restart"], "gateway-restart"), False
+
+
 @app.post("/api/gateway/restart")
 async def restart_gateway():
     """Kick off a ``hermes gateway restart`` in the background."""
     try:
-        proc = _spawn_hermes_action(["gateway", "restart"], "gateway-restart")
+        proc, _reused = _spawn_gateway_restart()
     except Exception as exc:
         _log.exception("Failed to spawn gateway restart")
         raise HTTPException(status_code=500, detail=f"Failed to restart gateway: {exc}")
@@ -3757,13 +3774,18 @@ def _restart_gateway_after_telegram_onboarding() -> dict[str, Any]:
     restart failures so the UI can fall back to the existing manual banner.
     """
     try:
-        proc = _spawn_hermes_action(["gateway", "restart"], "gateway-restart")
+        proc, reused = _spawn_gateway_restart()
     except Exception as exc:
         _log.exception("Failed to auto-restart gateway after Telegram onboarding")
         return {
             "restart_started": False,
             "restart_error": str(exc),
         }
+    if reused:
+        _log.info(
+            "Telegram onboarding: reusing in-flight gateway restart (pid %s)",
+            proc.pid,
+        )
     return {
         "restart_started": True,
         "restart_action": "gateway-restart",