Fix stacking query parameter

Multiple clicks on the login button added the next query parameter again.

Added logic to add the query parameter only once.

Closes #24 (closed)

portal/static/js/components/pieces/Nav.mjs

 import { NavPageLink } from "./NavPageLink.mjs"
 import { logout, isLoggedIn } from "../../account.mjs"
 
+const hasQueryParam = (param, path) => param in m.parsePathname(path).params
+
 export const Nav = {
     view() {
         return m("nav.box", [
@@ -60,7 +62,9 @@ export const Nav = {
                                       iconOnly: true,
                                   })
                                 : m(NavPageLink, {
-                                      href: `/login?next=${m.route.get() || "/"}`,
+                                      href: hasQueryParam("next", m.route.get())
+                                          ? m.route.get()
+                                          : `/login?next=${m.route.get() || "/"}`,
                                       label: " Anmelden",
                                       icon: "fa-solid fa-right-to-bracket",
                                   }),