fix(httpserver): bind port synchronously in OnStart so bind errors propagate

Previously ListenAndServe ran entirely in a goroutine, so a port-in-use
error was only logged and the launcher received nil from OnStart — leaving
the application running without an HTTP server.

Replace with net.Listen (synchronous) + srv.Serve(ln) (goroutine). A bind
failure now returns an error from OnStart, which the launcher treats as
fatal and triggers a clean shutdown immediately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

httpserver.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"net"
 	"net/http"
 	"time"
 
@@ -80,7 +81,10 @@ func (s *httpServer) OnInit() error {
 	return nil
 }
 
-// OnStart starts the HTTP server in a background goroutine.
+// OnStart binds the TCP listener synchronously so that a port conflict returns
+// an error immediately — allowing the launcher to trigger a clean shutdown
+// instead of running silently without an HTTP server. The accepted connections
+// are then served in a background goroutine.
 func (s *httpServer) OnStart() error {
 	host := s.cfg.Host
 	if host == "" {
@@ -90,16 +94,23 @@ func (s *httpServer) OnStart() error {
 	if port == 0 {
 		port = 8080
 	}
+	addr := fmt.Sprintf("%s:%d", host, port)
+
+	ln, err := net.Listen("tcp", addr)
+	if err != nil {
+		return fmt.Errorf("httpserver: bind %s: %w", addr, err)
+	}
+
 	s.srv = &http.Server{
-		Addr:         fmt.Sprintf("%s:%d", host, port),
+		Addr:         addr,
 		Handler:      s.Router,
 		ReadTimeout:  s.cfg.ReadTimeout,
 		WriteTimeout: s.cfg.WriteTimeout,
 		IdleTimeout:  s.cfg.IdleTimeout,
 	}
-	s.logger.Info("httpserver: starting", "addr", s.srv.Addr)
+	s.logger.Info("httpserver: starting", "addr", addr)
 	go func() {
-		if err := s.srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
+		if err := s.srv.Serve(ln); err != nil && !errors.Is(err, http.ErrServerClosed) {
 			s.logger.Error("httpserver: fatal error", err)
 		}
 	}()