Rework PostHog setup — use Next.js ingest proxy + posthog-js/react

Mirrors the proven pattern from gendersloty: rewrites /ingest/* to
the PostHog host so requests go through the same origin (no CORS,
no ad-blocker issues). Uses PostHogProvider and usePostHog from
posthog-js/react official React integration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/layout.tsx

@@ -4,6 +4,8 @@ import './globals.css'
 import Nav from '@/components/ui/Nav'
 import Footer from '@/components/ui/Footer'
 import PostHogProvider from '@/components/PostHogProvider'
+import { PostHogPageView } from '@/components/PostHogPageView'
+import { Suspense } from 'react'
 
 const cormorant = Cormorant_Garamond({
   subsets: ['latin'],
@@ -47,6 +49,9 @@ export default function RootLayout({
           }}
         />
         <PostHogProvider>
+          <Suspense fallback={null}>
+            <PostHogPageView />
+          </Suspense>
           <Nav />
           <main>{children}</main>
           <Footer />