chore: sync in-progress work across marketing, admin, API and tests

Snapshot of uncommitted work that had accumulated in the tree alongside
the Turnstile changes:

- marketing pages, SEO helpers (lib/seo.ts, lib/marketing/) and
  structured data
- admin billing actions and a per-user portfolio view, plus an admin
  error boundary
- rate limiting (lib/rate-limit.ts) applied across the /api/v1 surface
- CSP and proxy adjustments, accounting/webhook lib updates
- Playwright config and an e2e/unit test suite
- next bumped to ^16.3.4 with the lockfile regenerated
- generated AGENTS.md / CLAUDE.md

Authored by other sessions working in this tree; committed here so the
Turnstile work could be pushed without leaving the tree dirty.
Typecheck passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

lib/hooks/use-user.ts

@@ -12,43 +12,43 @@ interface UserState {
 
 export function useUser(): UserState {
   const { data: session, isPending } = useSession()
-  const [profile, setProfile] = useState<Profile | null>(null)
-  const [profileLoading, setProfileLoading] = useState(true)
+
+  // The fetched profile is stored together with the user id it belongs to, so
+  // "loaded" is DERIVED rather than tracked in a second state variable. That
+  // removes the synchronous setState in the effect body (which caused cascading
+  // renders) and, as a bonus, stops a previous user's profile from flashing
+  // while a new one loads.
+  const [fetched, setFetched] = useState<{ userId: string; profile: Profile | null } | null>(null)
 
   const userId = session?.user?.id
 
   useEffect(() => {
+    if (!userId) return
     let active = true
-    if (!userId) {
-      setProfile(null)
-      setProfileLoading(false)
-      return
-    }
-    setProfileLoading(true)
+
     fetch("/api/profile")
       .then((r) => (r.ok ? r.json() : { profile: null }))
       .then((data) => {
-        if (active) {
-          setProfile(data.profile ?? null)
-          setProfileLoading(false)
-        }
+        if (active) setFetched({ userId, profile: data.profile ?? null })
       })
       .catch(() => {
-        if (active) {
-          setProfile(null)
-          setProfileLoading(false)
-        }
+        if (active) setFetched({ userId, profile: null })
       })
+
     return () => {
       active = false
     }
   }, [userId])
 
+  const isCurrent = !!userId && fetched?.userId === userId
+
   return {
     user: session?.user
       ? { id: session.user.id, email: session.user.email, name: session.user.name }
       : null,
-    profile,
-    loading: isPending || profileLoading,
+    profile: isCurrent ? (fetched?.profile ?? null) : null,
+    // Signed out: nothing to load. Signed in: loading until this user's profile
+    // has actually come back.
+    loading: isPending || (!!userId && !isCurrent),
   }
 }