Add Move to Group context menu for session organization

[Copilot]

Groups could be created but sessions couldn't be moved into them—the plumbing existed (Groups and OnMove parameters) but no UI invoked it.

Changes

• SessionListItem.razor: Added "Move to…" submenu showing all groups except current. Only rendered when Groups.Count > 1.

• CopilotService.Organization.cs: Made MoveSession() defensive—auto-creates SessionMeta if missing (prevents silent no-ops for sessions not yet reconciled).

• SessionListItem.razor.css: Submenu styling—indented items, non-interactive label, proper hierarchy.

• SessionOrganizationTests.cs: 6 unit tests covering edge cases (missing meta, invalid groups, cross-group moves, event firing).

Example

// Before: silently failed if SessionMeta didn't exist
public void MoveSession(string sessionName, string groupId)
{
    var meta = Organization.Sessions.FirstOrDefault(m => m.SessionName == sessionName);
    if (meta != null) 
        meta.GroupId = groupId;
}

// After: creates meta on the fly
if (meta == null)
{
    meta = new SessionMeta { SessionName = sessionName, GroupId = groupId };
    Organization.Sessions.Add(meta);
}

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[PureWeen]

Hey there! 🌟

First off — amazing work identifying this gap and shipping a clean fix with tests. The defensive MoveSession() auto-creating SessionMeta on the fly was a really elegant solution, and those 6 unit tests covering the edge cases? Chef's kiss. 👨‍🍳

That said, the Force is already strong with this codebase — all of these changes have since been merged into main through other work. After rebasing, both this PR and its parent (#125) produce a zero diff against main, so there's nothing left to merge. These are not the diffs you're looking for. 👋

Closing this one out, but please keep those contributions coming! The quality of your code and test coverage is genuinely impressive. May the MoveSession be with you, always. ✨