Stop allowing cross-origin requests (closes #261)

The CORS middleware let every origin make credentialed requests, so a
page on another subdomain of the server's domain could read and act as
the signed-in user. Nothing needs cross-origin access, so the
middleware and the go-chi/cors dependency go. A test checks that a
request from another origin and a preflight get no
Access-Control-Allow-* headers.

Model: opus-5-5

internal/server/server_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"net/http/httptest"
 	"strconv"
 	"strings"
 	"testing"
@@ -188,6 +189,70 @@ func TestServeAfterStopDoesNotStopApp(t *testing.T) {
 	}
 }
 
+// TestNoCrossOriginAccess checks that the server allows no
+// cross-origin requests: neither a request from another origin
+// nor a preflight gets Access-Control-Allow-Origin or
+// Access-Control-Allow-Credentials, and a preflight gets 405
+// like any other OPTIONS request.
+func TestNoCrossOriginAccess(t *testing.T) {
+	var srv *server.Server
+
+	startApp(t, fx.Populate(&srv))
+
+	tests := []struct {
+		name          string
+		method        string
+		requestMethod string
+		status        int
+	}{
+		{
+			name:   "request",
+			method: http.MethodGet,
+			status: http.StatusOK,
+		},
+		{
+			name:          "preflight",
+			method:        http.MethodOptions,
+			requestMethod: http.MethodPost,
+			status:        http.StatusMethodNotAllowed,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			request := httptest.NewRequestWithContext(
+				t.Context(), test.method,
+				"http://chat.example.com/api/v1/server", nil,
+			)
+			request.Header.Set("Origin", "http://blog.example.com")
+
+			if test.requestMethod != "" {
+				request.Header.Set(
+					"Access-Control-Request-Method", test.requestMethod,
+				)
+			}
+
+			recorder := httptest.NewRecorder()
+
+			srv.ServeHTTP(recorder, request)
+
+			if recorder.Code != test.status {
+				t.Errorf("status %d, want %d", recorder.Code, test.status)
+			}
+
+			for _, header := range []string{
+				"Access-Control-Allow-Origin",
+				"Access-Control-Allow-Credentials",
+			} {
+				value := recorder.Header().Get(header)
+				if value != "" {
+					t.Errorf("%s is %q, want none", header, value)
+				}
+			}
+		})
+	}
+}
+
 // startApp starts the app newApp builds, with the HTTP port
 // set to 0, so the system gives the server a free port.
 func startApp(