fix(web): refuse cross-origin non-safe requests at the router (#48)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/web/handlers.go

@@ -229,7 +229,12 @@ func (a *App) Router() http.Handler {
 	// resolved user_id. /register lives inside the auth guard but is exempt from
 	// the registration gate (you must be able to reach it before you have a user).
 	root.Handle("/", a.Auth.Middleware(a.registrationGate(app)))
-	return root
+
+	// The session cookie is SameSite=Lax, which still travels on a POST made from a
+	// sibling *.d-ma.be page (same site, different origin). Refuse any non-safe request
+	// whose Origin or Sec-Fetch-Site says it did not come from this origin (#48). It wraps
+	// everything, so a route added later is covered without remembering to ask.
+	return http.NewCrossOriginProtection().Handler(root)
 }
 
 // handleWelcome renders the public landing page (/welcome). It is mounted outside