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_tuic_shutdown_test.go

@@ -74,7 +74,7 @@ func TestGracefulShutdownPersistsFinalTuicTraffic(t *testing.T) {
 		t.Fatal("TUIC listener did not start")
 	}
 
-	if err := NewServer().Stop(); err != nil {
+	if err := NewServer(nil).Stop(); err != nil {
 		t.Fatalf("graceful server shutdown: %v", err)
 	}