In-product expansion loops: upgrade nudges + invite-landing agent stats

[brianorwhatever]

From docs/business-plan.md §7 Phase 2.

Plan limits are already enforced server-side (requirePlan in convex/billing.ts) — make the upsell moments gracious, not a wall.

• Upgrade nudge when a user hits 5 lists or 3 collaborators (Free limits): explain, show what Pro unlocks, one-click checkout
• Invite landing page (src/pages/InviteLanding.tsx): surface the inviter's agent activity — "Ralph checked off 14 items this week — see how"
• Track nudge impression → upgrade conversion in PostHog
• Referral program promotion (convex/referrals.ts already grants temporary Pro): month of Pro both ways, surfaced post-activation and in Profile