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/node_contract_test.go

@@ -43,7 +43,7 @@ func startContractNode(t *testing.T) *nodeUnderContract {
 	t.Cleanup(func() { runtime.SetManager(prevMgr) })
 
 	previous := global.GetWebServer()
-	s := NewServer()
+	s := NewServer(nil)
 	s.cron = cron.New(cron.WithLocation(time.Local), cron.WithSeconds())
 	global.SetWebServer(s)
 	t.Cleanup(func() {