Migrate /agent to Next App Router [DO-NOT-MERGE]

[Strajk]

Not ready for merging!
Draft to discuss the approach.
More info on Discord 👋

[Strajk]

• improved file naming
  • app/_lib/supabase.tsx -> app/_lib/SupabaseProvider.tsx
  • app/_lib/posthog.tsx -> app/_lib/PostHogProvider.tsx
• switched order of providers: PostHogProvider > SupabaseProvider to SupabaseProvider > PostHogProvider, to have access to user in PostHogProvider
• dynamic loading of UI components in agent/.../setup to avoid hydration errors, more info in code comments
• reverted components/Layout, i opted to avoid changing unrelated components in this PR
• collocated all tracking (Posthog, GA) to template.tsx, as recommended in Next.js docs
• added export const dynamic = "force-dynamic" to routes using reading cookies
• got rid of default exports in favour of named exports, for consistency
• added encoding package as dev dep, as recommended here examples: export force-dynamic from all dynamic routes vercel/next.js#52916
• ran next build to check for errors
• checked console for posthog tracking events, seems allrighty

[Strajk]