Add thread-safety locks for Organization state access

[Copilot]

Organization.Sessions and Organization.Groups are List<T> collections accessed from both the UI thread (Blazor event handlers) and background threads (WebSocket receive loop in WsBridgeServer.HandleOrganizationCommand), creating a race condition.

Changes:

• Added _organizationLock to synchronize all accesses to Organization.Sessions and Organization.Groups
• Protected all 17 methods in CopilotService.Organization.cs that read/write organization state
• Protected 4 direct access points in CopilotService.cs (ResumeSessionAsync, CreateSessionAsync, CreateSession, RenameSession)

Lock patterns:

• Snapshot saves: Serialize under lock, write file outside lock to avoid holding lock during I/O
• GetOrganizedSessions: Copy state under lock, sort outside lock for better performance
• C# locks are reentrant, so nested calls (LoadOrganization → ReconcileOrganization → SaveOrganization) work correctly

public void MoveSession(string sessionName, string groupId)
{
    lock (_organizationLock)
    {
        if (!Organization.Groups.Any(g => g.Id == groupId))
            return;

        var meta = Organization.Sessions.FirstOrDefault(m => m.SessionName == sessionName);
        // ... update meta, save, notify
    }
}

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