fix(chat): KeyNotFoundException dans HubConnectionManager.OnConnected

ChatUserNames n'etait jamais alimente (relique d'un SetUserName disparu),
ce qui faisait lever KeyNotFoundException a chaque connexion SignalR et
fermait immediatement la connexion cote client ("Déconnecté (The server
closed the connection...)" dans PostIt).

- IConnexionManager.OnConnected prend desormais le userName en parametre
- HubConnectionManager.OnConnected alimente ChatUserNames et ChatCxIds
- ChatHub.OnConnectedAsync passe le userName resolu depuis les claims

Inclut aussi les changements PostIt lies (bandeau de statut avec
commande d'action, handlers Reconnecting/Reconnected/Closed).

src/Yavsc.Server/Hubs/ChatHub.cs

@@ -90,7 +90,7 @@ namespace Yavsc.Server.Hubs
             {
                 await Groups.AddToGroupAsync(Context.ConnectionId, ChatHubConstants.HubGroupFollowingPrefix + uid);
             }
-            _cxManager.OnConnected(Context.ConnectionId, isCop);
+            _cxManager.OnConnected(Context.ConnectionId, userName, isCop);
             await base.OnConnectedAsync();
         }
 

src/Yavsc.Server/Interfaces/IConnexionManager.cs

@@ -4,7 +4,7 @@ namespace Yavsc.Services
 {
     public interface IConnexionManager {
         string GetUserName (string cxId);
-        void OnConnected(string cxId, bool isCop);
+        void OnConnected(string cxId, string userName, bool isCop);
         bool IsConnected(string candidate);
 
         void OnDisconnected (string cxId);

src/Yavsc.Server/Services/HubConnectionManager.cs

@@ -53,13 +53,15 @@ namespace Yavsc.Services
             _localizer = stringLocFactory.Create(typeof(HubConnectionManager));
         }
 
-        // Username must have been set before calling this method.
-        public void OnConnected(string cxId,  bool isCop)
+        public void OnConnected(string cxId, string userName, bool isCop)
         {
-            var username = ChatUserNames[cxId];
-            if (!IsConnected(username))
-                ChatRoomPresence[username] = new List<string>();
-            _isCop[username] = isCop;
+            ChatUserNames[cxId] = userName;
+            ChatCxIds.AddOrUpdate(userName,
+                _ => new List<string> { cxId },
+                (_, list) => { lock (list) { if (!list.Contains(cxId)) list.Add(cxId); } return list; });
+            if (!IsConnected(userName))
+                ChatRoomPresence[userName] = new List<string>();
+            _isCop[userName] = isCop;
         }
 
         public bool IsConnected(string candidate)