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Diagnose and fix Next.js dev-mode issues with the blog at apps/blog/blog. Use when the blog dev server returns 500s, when a published post 404s, when tables or code blocks render without their styling (no borders, no background), when the page is white/blank with a collapsed layout, when a Playwright screenshot of a working page comes back blank, or when "Cannot read properties of undefined (reading 'contentHtml')" appears in the dev server log. Also use when Playwright MCP refuses to launch with "Browser is already in use".