[codex] fix login screen layout

[rogerchappel]

Summary

• Render public chrome-less routes without the authenticated app shell.
• Prevent the login page from being constrained to the collapsed sidebar grid column.

Root cause

The global AppShell always rendered the sidebar grid wrapper. On /, Sidebar returned null, but its collapse effect still left the grid using the 80px collapsed sidebar column, so the login form collapsed into a narrow strip on the left side.

Validation

• npm run typecheck
• npm run lint:check -- src/components/app-shell.tsx (passes with existing repo warnings)
• Playwright layout check at desktop and mobile viewports confirmed the login card is centered and full width within its max width.
• Pre-push hook ran tsc --noEmit and next build successfully.