#419 fix: PostHog cross-subdomain tracking and cookie consent

Summary

• Configure PostHog for cross-subdomain user tracking with persistence: "cookie" and persistence_name
• Add cookie domain config to all apps (web, marketing, admin, docs, desktop)
• Create PostHogUserIdentifier component to identify users to PostHog after auth
• Add Trigger.dev-style cookie consent banner to marketing site with opt-out model
• Add Next.js 16 proxy.ts documentation note to AGENTS.md

Changes

• All apps now use consistent PostHog initialization with cross-subdomain tracking
• PostHogUserIdentifier uses Clerk (web apps) or tRPC (desktop) internally to get user data
• Marketing site shows small bottom-left cookie banner with Accept/Opt-out options
• Default NEXT_PUBLIC_COOKIE_DOMAIN to localhost for development

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Integrated analytics across apps with automatic user identification (syncs account info to analytics when signed in).
  • Added a dismissible cookie consent banner in Marketing to accept or opt out of analytics.
• Chores
  • Configured analytics routing/endpoints and app-level providers; added public analytics keys and cookie name for consistent behavior across deployments.

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