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Step-by-step fixes for the errors that stop apps built with Lovable, Bolt, Replit and Cursor. Each guide explains the error in plain words and gives the exact fix.

• Lovable app: blank page or Vercel 404 after deployYour Lovable app works in preview but shows a blank page, a Vercel 404 NOT_FOUND on refresh, or a failed build after deploy. The four causes and the exact fix for each, for Vercel, Netlify and Cloudflare Pages.
• Stripe webhook secret: where to find it, and fixing 400/401 errorsWhere to find your Stripe webhook signing secret (whsec_), and how to fix webhooks that fail in Supabase Edge Functions: 'No signatures found matching the expected signature', 401 Missing authorization header, test vs live secrets.
• Supabase: "Error sending confirmation email"New users can't sign up because Supabase won't send the confirmation email, or says 'Email address not authorized'. Why Supabase's built-in email stops working at launch, and how to connect a real email provider.
• Supabase login redirects to localhost after deployGoogle sign-in or the email link sends your users to localhost:3000 or an old preview after you deploy. Fix the Site URL, the Redirect URLs list, your code and Google's callback in four steps.
• Supabase RLS: "new row violates row-level security policy"Fix 'new row violates row-level security policy for table', empty data after login, and the 'RLS Disabled in Public' warning in Supabase apps built with Lovable, Bolt or Replit: the real causes, the exact SQL, and the fix you must never use.
• Security checklist for AI-built apps before you launchEight checks that catch the holes found again and again in apps built with Lovable, Bolt, Replit and Cursor: Supabase RLS, leaked keys, users who can make themselves admin, unsigned payments, open storage. How to check each in minutes.