Upgrade frontend dependencies and switch ESLint to a flat config

- Runtime majors: vue-router 5, reveal.js 6, fontawesome 7. Checked against the
  code that uses them: no change needed for reveal.js (the default export is
  still a callable factory and every method/option used is still present) or
  fontawesome (all seven registered icons still resolve).
- frontend/src/router/index.js: guards return the redirect instead of calling the
  deprecated next() callback, which works on vue-router 4 and 5.
- Add frontend/eslint.config.mjs because eslint 10 requires a flat config; the
  lint script now runs eslint directly instead of vue-cli-service.
- Drop tooling nothing in the webpack configs uses (@vue/cli-*, jshint,
  html-webpack-plugin, webpack-bundle-analyzer, @babel/core) and delete the dead
  vue.config.js. npm audit 35 -> 5 (all dev-only), --omit=dev is 0.
- frontend/src/views/UserView.vue: the $route watcher no longer declares unused
  params.

frontend/src/router/index.js

@@ -10,26 +10,22 @@ const LoginView = () => import('@/views/LoginView')
 const HistoryView = () => import('@/views/HistoryView')
 
 // Navigation guard to check if user is authenticated
-function requireAuth(to, from, next) {
+function requireAuth(to) {
   if (!store.getters.isAuthenticated) {
-    next({
+    return {
       name: 'login',
       query: { next: to.fullPath, error: 'Please log in to access this page' }
-    });
-  } else {
-    next();
+    };
   }
 }
 
 // Navigation guard to check if user is admin
-function requireAdmin(to, from, next) {
+function requireAdmin(to) {
   if (!store.getters.isAuthenticated || !store.getters.is_superuser) {
-    next({
+    return {
       name: 'login',
       query: { next: to.fullPath, error: 'Admin access required' }
-    });
-  } else {
-    next();
+    };
   }
 }
 

frontend/src/views/UserView.vue

@@ -67,7 +67,7 @@ export default {
     }
   },
   watch: {
-    $route(to, from) {
+    $route() {
       this.updateUsers()
       this.crumbs = [...default_crumbs]
       if (this.userid){