fix(web): install the share-link provider before the panel serves (#6914)

The panel started answering requests before the share-link generator was
registered, so an "all links", client sub-links or Telegram link request that
arrived during start-up got "sub link provider not registered". NewServer now
takes the provider and installs it before anything listens, and the provider
registry is guarded by a lock, since start-up writes it while requests read it.

internal/web/web_sublink_test.go

@@ -0,0 +1,40 @@
+package web
+
+import (
+	"testing"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
+	"github.com/mhsanaei/3x-ui/v3/internal/web/service"
+)
+
+type fakeSubLinkProvider struct {
+	links []string
+}
+
+func (f fakeSubLinkProvider) SubLinksForSubId(host, subId string) ([]string, error) {
+	return f.links, nil
+}
+
+func (f fakeSubLinkProvider) LinksForClient(host string, inbound *model.Inbound, email string) []string {
+	return f.links
+}
+
+func (f fakeSubLinkProvider) LinksForInbounds(host string, inbounds []*model.Inbound) []string {
+	return f.links
+}
+
+// A link request can arrive as soon as the panel serves, so NewServer must install the provider.
+func TestNewServerInstallsTheShareLinkProviderBeforeServing(t *testing.T) {
+	service.RegisterSubLinkProvider(nil)
+	t.Cleanup(func() { service.RegisterSubLinkProvider(nil) })
+
+	NewServer(fakeSubLinkProvider{links: []string{"vless://example"}})
+
+	links, err := (&service.InboundService{}).GetSubLinks("panel.example", "subid")
+	if err != nil {
+		t.Fatalf("the share-link provider must be installed before the panel serves: %v", err)
+	}
+	if len(links) != 1 || links[0] != "vless://example" {
+		t.Fatalf("links = %v, want the provider's links", links)
+	}
+}