Feat/centralize url handling

[bjornpagen]

feat: simplify URL handling with Next.js headers

• Remove BASE_URL and NEXT_PUBLIC_BASE_URL environment variables
• Add baseUrl() utility using Next.js headers() to dynamically get host
• Update Stripe redirect URLs to use baseUrl()
• Remove URL configuration from setup script

This change eliminates environment variables in favor of dynamically detecting the correct URL at request time. Works automatically across all environments: local development (http), Vercel deployments (https), and custom domains.

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