Upgrade ESLint to v10 flat config setup

[Codex]

ESLint was on v9 and flat-config migration was incomplete, blocking the bump to 10.1.0.

• Dependencies: bump eslint to 10.1.0, add @eslint/js, @eslint/eslintrc, vue-eslint-parser, upgrade eslint-plugin-vue to 10.8.0 to match ESLint 10.
• Config: replace FlatCompat usage with native flat config; apply Vue flat essential preset + Prettier flat; set browser/node globals explicitly; move ignores into eslint.config.js and drop obsolete .eslintignore.
• Cleanups: remove /* eslint-env */ header in quasar.config.js; remove unused disable in postcss.config.cjs.

Example of updated flat config:

const globals = require("globals");
const prettier = require("eslint-config-prettier/flat");
const vue = require("eslint-plugin-vue");

module.exports = [
  { ignores: ["dist/**", ".quasar/**", ".eslintrc.cjs"] },
  {
    languageOptions: {
      ecmaVersion: 2021,
      sourceType: "module",
      globals: { ...globals.browser, ...globals.node },
    },
    plugins: { vue },
  },
  ...vue.configs["flat/essential"],
  prettier,
  { rules: { "vue/no-v-text-v-html-on-component": "off" } },
];