fix(shutdown): defer reclose via Dispatcher so OnClosing yields first

[AThraen]

Summary

• Fixes InvalidOperationException: Cannot ... call Close ... while a Window is closing. thrown from MainWindow.OnClosing at line 4770 when shutting down in --clean debug mode.
• Queues the post-cleanup reclose via Dispatcher.BeginInvoke instead of calling Close() inline, so WPF gets a chance to process the cancelled first pass and reset its internal _isClosing flag.

Root cause

OnClosing is async void using a cancel-then-reclose pattern: first entry sets e.Cancel = true, runs async cleanup, then calls Close() to re-enter through the _shutdownComplete branch. The cancellation only takes effect once OnClosing yields control back to WPF — that's when WPF reads e.Cancel and resets _isClosing.

In --clean mode none of the awaits actually yield:

• SaveStateAsync() short-circuits (if (App.CleanStart) return;), so its await resolves synchronously
• The foreach over _vm.Sessions is empty because clean mode clears the session list, so await DisposeAndWaitForExitAsync never runs
• The remaining DB close / tray dispose is synchronous

So OnClosing runs straight through to the final Close() without ever yielding — WPF's _isClosing is still true and Close() throws.

Fix

Swap inline Close() for _ = Dispatcher.BeginInvoke(new System.Action(Close)). OnClosing returns first, WPF resets _isClosing, then the queued Close() re-enters cleanly through the _shutdownComplete branch. Works in both the no-yield clean case and the normal case with live sessions.

Test plan

• Run dotnet run --project src/CodeShellManager/CodeShellManager.csproj -- --clean → close window → verify no exception in crash.log and the app exits cleanly.
• Run without --clean with several live sessions (including a Claude session) → close window → verify normal shutdown, state.json is written, and Claude sessions are disposed serially.
• Double-click the X during async cleanup → verify the second OnClosing entry still hits the _isShuttingDown gate and doesn't re-run cleanup.

🤖 Generated with Claude Code