feat: registration_enabled admin setting — hides signup surfaces, gates the API

One boolean (Security > Public Registration) switches the whole public
signup lifecycle: front page + login links, the logged-out sidebar's
Create-an-account (via a package-level templates hook so shared page
templates keep their signatures), GET /register -> 302 /login, and an
early 403 on POST /api/auth/register. First-user exception preserved:
zero admins keeps every route and link reachable for bootstrapping —
same 'users exist' reasoning as the setup gate. Admin user creation is
unaffected by design.

Live E2E verified: flag off hides all surfaces and blocks the POST
(403), flag on restores them (201); the admin UI renders the row
automatically.

cmd/server/main.go

@@ -10,6 +10,7 @@ import (
 	"bookhoard/internal/router"
 	"bookhoard/internal/services"
 	"bookhoard/internal/sync"
+	"bookhoard/templates"
 	"context"
 	"log"
 	"time"
@@ -56,6 +57,20 @@ func main() {
 	if err := registry.Load(ctx); err != nil {
 		log.Printf("⚠️ Could not load system settings (using defaults): %v", err)
 	}
+
+	// Public signup surfaces follow the registration_enabled flag, with the
+	// first-user exception: an unconfigured server (zero admins) keeps its
+	// signup links and registration route reachable for bootstrapping.
+	templates.SetSignupVisibility(func() bool {
+		if registry.RegistrationEnabled() {
+			return true
+		}
+		count, err := queries.CountAdmins(context.Background())
+		if err != nil {
+			return true // fail open, matching setupstatus
+		}
+		return count == 0
+	})
 	// Wire the registry into the package-level password validator so live
 	// rule changes apply to the echo struct-tag validator and ValidatePassword.
 	middleware.SetDefaultPasswordSettings(registry)