feat(surf): print the serve URL and admin status on startup

- Add surf.Router.AdminPrefix (additive): the admin mount path and true
  when BuildRouter mounted the cabana admin, false otherwise
- serve binds with net.Listen before printing, so a busy port fails
  without a misleading URL, then prints the listen address with a
  browser URL and an admin status line: the admin URL, a warning naming
  summer admin:create when backend_users is empty or missing, or
  admin: off with the steps to add an admin controller
- The administrator count is bounded by a 2 s timeout and never fails boot
- route:list ends with the same admin status line
- surf and cabana READMEs and the console and routing docs describe the output

modules/surf/serve_test.go

@@ -1,12 +1,15 @@
 package surf
 
 import (
+	"bytes"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 
 	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/bonfire"
 	"git.golem15.com/golem15/summercms/modules/compass"
 	"gocloud.dev/blob"
 )
@@ -51,3 +54,71 @@ func serveTestApp(t *testing.T, storageYAML string) *backpack.App {
 	}
 	return backpack.New(cfg)
 }
+
+func TestServeURL(t *testing.T) {
+	for _, tc := range []struct{ addr, want string }{
+		{":8123", "http://127.0.0.1:8123/"},
+		{"127.0.0.1:9000", "http://127.0.0.1:9000/"},
+		{"[::1]:8080", "http://[::1]:8080/"},
+		{"0.0.0.0:80", "http://127.0.0.1/"},
+		{"[::]:8080", "http://127.0.0.1:8080/"},
+		{"localhost:3000", "http://localhost:3000/"},
+		{"8080", ""},
+		{"", ""},
+		{"127.0.0.1:", ""},
+	} {
+		if got := serveURL(tc.addr); got != tc.want {
+			t.Errorf("serveURL(%q) = %q, want %q", tc.addr, got, tc.want)
+		}
+	}
+}
+
+func TestServeBanner(t *testing.T) {
+	listen := bannerLine{text: "listening on :8123, open http://127.0.0.1:8123/"}
+	mounted := bannerLine{text: "admin: http://127.0.0.1:8123/backend"}
+	create := "summer admin:create --email you@example.com --superuser"
+	for _, tc := range []struct {
+		name    string
+		addr    string
+		prefix  string
+		mounted bool
+		admins  adminPresence
+		want    []bannerLine
+	}{
+		{"present", ":8123", "/backend", true, adminsPresent, []bannerLine{listen, mounted}},
+		{"none", ":8123", "/backend", true, adminsNone, []bannerLine{listen, mounted,
+			{warn: true, text: "no administrators yet; create one with " + create}}},
+		{"missing table", ":8123", "/backend", true, adminsMissingTable, []bannerLine{listen, mounted,
+			{warn: true, text: "the admin tables do not exist yet; run summer migrate, then " + create}}},
+		{"unknown", ":8123", "/backend", true, adminsUnknown, []bannerLine{listen, mounted,
+			{text: "sign in with an administrator; " + create + " creates one"}}},
+		{"off", ":8123", "", false, adminsUnknown, []bannerLine{listen,
+			{text: "admin: off — no plugin registers an admin controller"},
+			{text: "to add one, run summer make:admin-controller <vendor.plugin> <Name>, then summer build, summer migrate and " + create + "; the admin needs SUMMER_ADMIN__JWT__SECRET, which summer new writes to .env"}}},
+		{"unparsable address", "8080", "/backend", true, adminsPresent, []bannerLine{
+			{text: "listening on 8080"}, {text: "admin: mounted at /backend"}}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			got := serveBanner(tc.addr, tc.prefix, tc.mounted, tc.admins)
+			if !slices.Equal(got, tc.want) {
+				t.Fatalf("serveBanner:\n got %#v\nwant %#v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestPrintBanner(t *testing.T) {
+	var buf bytes.Buffer
+	out := bonfire.NewOutput(strings.NewReader(""), &buf, &buf)
+	printBanner(out, []bannerLine{{text: "plain"}, {warn: true, text: "careful"}})
+	got := buf.String()
+	if !strings.Contains(got, "ℹ plain") || !strings.Contains(got, "⚠ careful") {
+		t.Fatalf("printBanner output = %q", got)
+	}
+}
+
+func TestCountAdminsNilDB(t *testing.T) {
+	if got := countAdmins(t.Context(), nil); got != adminsUnknown {
+		t.Fatalf("countAdmins(nil) = %v, want adminsUnknown", got)
+	}
+}