feat(setup): add web-based first-run configuration wizard

When config.json is missing, Erupe now launches a temporary HTTP server
on port 8080 serving a guided setup wizard instead of exiting with a
cryptic error. The wizard walks users through database connection,
schema initialization (pg_restore + SQL migrations), and server settings,
then writes config.json and continues normal startup without restart.

server/setup/setup.go

@@ -0,0 +1,55 @@
+package setup
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/gorilla/mux"
+	"go.uber.org/zap"
+)
+
+// Run starts a temporary HTTP server serving the setup wizard.
+// It blocks until the user completes setup and config.json is written.
+func Run(logger *zap.Logger, port int) error {
+	ws := &wizardServer{
+		logger: logger,
+		done:   make(chan struct{}),
+	}
+
+	r := mux.NewRouter()
+	r.HandleFunc("/", ws.handleIndex).Methods("GET")
+	r.HandleFunc("/api/setup/detect-ip", ws.handleDetectIP).Methods("GET")
+	r.HandleFunc("/api/setup/client-modes", ws.handleClientModes).Methods("GET")
+	r.HandleFunc("/api/setup/test-db", ws.handleTestDB).Methods("POST")
+	r.HandleFunc("/api/setup/init-db", ws.handleInitDB).Methods("POST")
+	r.HandleFunc("/api/setup/finish", ws.handleFinish).Methods("POST")
+
+	srv := &http.Server{
+		Addr:    fmt.Sprintf(":%d", port),
+		Handler: r,
+	}
+
+	logger.Info(fmt.Sprintf("Setup wizard available at http://localhost:%d", port))
+	fmt.Printf("\n  >>> Open http://localhost:%d in your browser to configure Erupe <<<\n\n", port)
+
+	// Start the HTTP server in a goroutine.
+	errCh := make(chan error, 1)
+	go func() {
+		if err := srv.ListenAndServe(); err != http.ErrServerClosed {
+			errCh <- err
+		}
+	}()
+
+	// Wait for either completion or server error.
+	select {
+	case <-ws.done:
+		logger.Info("Setup complete, shutting down wizard")
+		if err := srv.Shutdown(context.Background()); err != nil {
+			logger.Warn("Error shutting down wizard server", zap.Error(err))
+		}
+		return nil
+	case err := <-errCh:
+		return fmt.Errorf("setup wizard server error: %w", err)
+	}
+}