fix(astro): don't modify state during re-renders of `<WorkspacePanelWrapper />`

[AriPerkkio]

Fixes React's state warning that happen when navigating between pages. setLesson that's done inside rendering block attempts to update state of the components that are just about to unmount.

Warning: Cannot update a component (`PreviewPanel`) while rendering a different component (`WorkspacePanelWrapper`). To locate the bad setState() call inside `WorkspacePanelWrapper`, follow the stack trace as described in https://reactjs.org/link/setstate-in-render
    at WorkspacePanelWrapper (http://localhost:4321/@fs/Users/x/tutorialkit/packages/astro/dist/default/components/WorkspacePanelWrapper.tsx:23:41)

printWarning                        @    react-dom.development.js:86
error                               @    react-dom.development.js:60
warnAboutRenderPhaseUpdatesInDEV    @    react-dom.development.js:27531
scheduleUpdateOnFiber               @    react-dom.development.js:25537
forceStoreRerender                  @    react-dom.development.js:16158
handleStoreChange                   @    react-dom.development.js:16134
notify                              @    index.js:55
set                                 @    index.js:72
setPreviews                         @    previews.js:77
setLesson                           @    index.js:96
WorkspacePanelWrapper               @    WorkspacePanelWrapper.tsx:15

[stackblitz[bot]]

Run & review this pull request in StackBlitz Codeflow.