[billing] Redesign pricing page for per-site model

- Rewrite `PricingPage.tsx` with per-site calculator

- "How many sites do you need?" slider showing monthly total

- First site full price, additional sites at 50%

- Lifetime Pro section: early bird 399 / regular 499, 2 Pro sites included

**Files:** `src/components/landing/PricingPage.tsx`