Subscription funnel improvements and study page SEO

[vishakh]

• Subscribe page now shows a feature comparison (Premium vs. free) before the auth gate, so unauthenticated users understand what they are paying for before being asked to sign in.
• Premium gates in DNA Chat and Analyze now redirect all non-subscribers to /subscribe instead of opening the auth modal. Previously, unauthenticated users never saw the subscribe page or its feature list.
• The Research button in DNA Chat is now enabled for non-subscribers. Clicking it redirects to /subscribe. Previously it was disabled and gave no indication of how to unlock it.
• Subscribe page copy updated: product name corrected to "Monadic DNA Explorer Premium", privacy reassurance added to the sign-in prompt, active subscriber card updated to reflect all unlocked features.
• Study detail pages are now server-rendered. Previously the page was a client component that fetched data via useEffect, so search crawlers saw an empty shell. Now the study title, trait, author, statistics, and SNP data are in the initial HTML. Personal result banner and inline analysis remain client-side.
• pg added to serverExternalPackages and Node.js built-ins (dns, net, tls) added to the client webpack fallback to prevent bundling errors from the new server component.