fix: keep the page gutter beside the sidebar

[AmrMsCLL]

What changed

When AppShell has a sidebar, main was the flex item that filled the row. flex-1 then overrode the shell width, so the page lost its side margins: it touched the screen edge on phones and the sidebar on desktop. A wrapper is now the flex item, and main keeps the shell width and centring.

Found while running Strata's first build on a local stack at 375 px and on desktop.

Verification

• The AppShell spec now checks that main sits inside the wrapper, keeps shell and has no flex-1.
• npm run lint, npm test (33 tests) and npm run build pass.

Checklist

• The change is focused and contains no credentials or generated secrets.
• Documentation and tests were updated when behavior changed.
• Deployment or rollback considerations are described when relevant. Nothing deploys; 0.2.1 follows.