Upgrade eslint tooling to v10 and fix linting bugs (#601)

- Upgrade eslint 9→10, eslint-config-prettier 9→10, eslint-plugin-vue 9→10
- Upgrade @types/node 20→24
- Migrate from legacy .eslintrc.mjs to flat config eslint.config.mjs
- Remove --ext flag from npm test script (dropped in eslint 10)
- Fix vue/no-ref-as-operand: add missing .value to ref assignments (5 files)
- Fix vue/return-in-computed-property: add default returns (2 files)
- Fix vue/no-side-effects-in-computed-properties: move mutation to watcher
- Fix vue/no-dupe-keys: remove ref shadowing prop in DomPDFDriver
- Fix vue/no-deprecated-slot-attribute: migrate to v-slot syntax (3 files)
- Fix vue/require-valid-default-prop: use factory function for array default
- Fix vue/no-unused-vars: remove unused slot destructure
- Disable vue/no-mutating-props (false positive for Pinia store props)

eslint.config.mjs

@@ -0,0 +1,13 @@
+import pluginVue from 'eslint-plugin-vue'
+import eslintConfigPrettier from 'eslint-config-prettier'
+
+export default [
+  ...pluginVue.configs['flat/recommended'],
+  eslintConfigPrettier,
+  {
+    files: ['resources/scripts/**/*.{js,vue}'],
+    rules: {
+      'vue/no-mutating-props': 'off',
+    },
+  },
+]