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/eslint.config.mjs

@@ -0,0 +1,43 @@
+import js from '@eslint/js'
+import pluginVue from 'eslint-plugin-vue'
+
+export default [
+  { ignores: ['dist/**', 'node_modules/**', 'public/**'] },
+  js.configs.recommended,
+  ...pluginVue.configs['flat/essential'],
+
+  // Build scripts (webpack.*.js) run in Node, so they use CommonJS globals.
+  {
+    files: ['**/*.js'],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: 'commonjs',
+      globals: {
+        require: 'readonly',
+        module: 'readonly',
+        __dirname: 'readonly',
+        process: 'readonly',
+        console: 'readonly',
+      },
+    },
+  },
+
+  // Application code is browser code bundled by webpack.
+  {
+    files: ['src/**'],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: 'module',
+      globals: {
+        window: 'readonly',
+        document: 'readonly',
+        localStorage: 'readonly',
+        console: 'readonly',
+        setTimeout: 'readonly',
+        clearTimeout: 'readonly',
+        process: 'readonly',
+      },
+    },
+    rules: {},
+  },
+]