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/service/sublink_registry_test.go

@@ -0,0 +1,44 @@
+package service
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
+)
+
+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
+}
+
+// Startup writes the provider registry while link requests read it; under -race an
+// unguarded registry fails here.
+func TestSubLinkProviderRegistrationIsRaceFree(t *testing.T) {
+	t.Cleanup(func() { RegisterSubLinkProvider(nil) })
+
+	var wg sync.WaitGroup
+	for range 8 {
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			RegisterSubLinkProvider(fakeSubLinkProvider{links: []string{"vless://example"}})
+		}()
+		go func() {
+			defer wg.Done()
+			_, _ = (&InboundService{}).GetSubLinks("panel.example", "subid")
+		}()
+	}
+	wg.Wait()
+}