[ci] add stylelint dependency and minimal config (#5926)

## Summary

Adds [stylelint](https://stylelint.io/) configuration and tooling to
enforce CSS/SCSS code quality and consistency across Vue components.

Starts with 21 focused rules for linting CSS in Vue SFC files and
standalone stylesheets. Configuration uses postcss-html to parse Vue
`<style>` blocks and includes whitelists for Tailwind v4 at-rules
(`@reference`, `@plugin`, `@custom-variant`, `@utility`) and
Electron-specific CSS properties (`speak: none`, `app-region`). Rules
emphasize modern CSS syntax (numeric font weights, modern color
functions, double-colon pseudo-elements) while avoiding overly
opinionated rules like hex color length enforcement (for now).

Currently finds 113 issues (79% auto-fixable). This PR only adds the
tooling via `pnpm stylelint` and `pnpm stylelint:fix` scripts - no
pre-commit hooks or CI integration yet. A follow-up PR will auto-fix the
fixable issues and optionally add enforcement to the commit workflow.

## Changes

- **What**: Integrated [stylelint](https://stylelint.io/) with Vue.js
support via postcss-html parser
- **Dependencies**: Added `stylelint@16.24.0`, `postcss-html@1.8.0`

## Review Focus

CSS rule strictness and Tailwind CSS compatibility - particularly the
`no-descending-specificity` rule and Tailwind-specific function ignores.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5926-ci-add-stylelint-dependency-and-minimal-config-2836d73d3650813ea7b7eb714ba7748a)
by [Unito](https://www.unito.io)

pnpm-workspace.yaml

@@ -39,7 +39,9 @@ catalog:
   'eslint-plugin-vue': ^10.4.0
   globals: ^15.9.0
   '@intlify/eslint-plugin-vue-i18n': ^4.1.0
+  postcss-html: ^1.8.0
   prettier: ^3.3.2
+  stylelint: ^16.24.0
   'typescript-eslint': ^8.44.0
   'vue-eslint-parser': ^10.2.0