feat(listsessions): add join/leave buttons for players

For non-managers /listsessions now shows player-friendly actions:
- ✅ Записаться <date> when not registered
- ✖️ Выйти <date> when already active
- ✖️ Выйти из ожидания <date> when waitlisted

Extend SessionListItemDto and the shared SQL query with IsUserActive
and IsUserWaitlisted flags so the renderer can choose the right button.
Update tests to cover all three player states.

src/GmRelay.Shared/Features/Sessions/ListSessions/ListSessionsHandler.cs

@@ -5,7 +5,17 @@ using Npgsql;
 
 namespace GmRelay.Shared.Features.Sessions.ListSessions;
 
-public sealed record SessionListItemDto(Guid Id, string Title, DateTime ScheduledAt, string Status, int? MaxPlayers, int PlayerCount, int WaitlistCount, bool CanManage);
+public sealed record SessionListItemDto(
+    Guid Id,
+    string Title,
+    DateTime ScheduledAt,
+    string Status,
+    int? MaxPlayers,
+    int PlayerCount,
+    int WaitlistCount,
+    bool CanManage,
+    bool IsUserActive,
+    bool IsUserWaitlisted);
 
 public sealed record SessionListResult(
     IReadOnlyList<SessionListItemDto> Sessions,
@@ -29,7 +39,27 @@ public sealed class ListSessionsHandler(
                          WHERE gm.group_id = s.group_id
                            AND manager_player.platform = @Platform
                            AND manager_player.external_user_id = @ExternalUserId
-                     ) AS CanManage
+                     ) AS CanManage,
+                     EXISTS (
+                         SELECT 1
+                         FROM session_participants user_sp
+                         JOIN players user_p ON user_p.id = user_sp.player_id
+                         WHERE user_sp.session_id = s.id
+                           AND user_sp.is_gm = false
+                           AND user_sp.registration_status = @Active
+                           AND user_p.platform = @Platform
+                           AND user_p.external_user_id = @ExternalUserId
+                     ) AS IsUserActive,
+                     EXISTS (
+                         SELECT 1
+                         FROM session_participants user_sp
+                         JOIN players user_p ON user_p.id = user_sp.player_id
+                         WHERE user_sp.session_id = s.id
+                           AND user_sp.is_gm = false
+                           AND user_sp.registration_status = @Waitlisted
+                           AND user_p.platform = @Platform
+                           AND user_p.external_user_id = @ExternalUserId
+                     ) AS IsUserWaitlisted
               FROM sessions s
               JOIN game_groups g ON s.group_id = g.id
               LEFT JOIN session_participants sp ON s.id = sp.session_id