Write a form-error page's status only after it renders (closes #128)

The login form and the webhook create and edit forms set their 400 or
409 with WriteHeader before calling renderTemplate, so a template
failure there answered with that status instead of 500. A variant of
renderTemplate now takes the status and writes it only once the page
has rendered into the buffer; renderTemplate sends 200 through it.

Model: opus-5-5

internal/handlers/auth.go

@@ -139,8 +139,7 @@ func (h *Handlers) renderLoginError(
 		),
 	}
 
-	w.WriteHeader(status)
-	h.renderTemplate(w, r, "login.html", data)
+	h.renderTemplateStatus(w, r, "login.html", data, status)
 }
 
 // authenticateUser looks up and verifies a user's credentials.