[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)

package.json

@@ -35,6 +35,8 @@
     "prepare": "husky || true && git config blame.ignoreRevsFile .git-blame-ignore-revs || true",
     "preview": "nx preview",
     "storybook": "nx storybook -p 6006",
+    "stylelint:fix": "stylelint --cache --fix",
+    "stylelint": "stylelint --cache",
     "test:browser": "pnpm exec nx e2e",
     "test:unit": "nx run test",
     "typecheck": "vue-tsc --noEmit",
@@ -79,8 +81,10 @@
     "knip": "catalog:",
     "lint-staged": "catalog:",
     "nx": "catalog:",
+    "postcss-html": "catalog:",
     "prettier": "catalog:",
     "storybook": "catalog:",
+    "stylelint": "catalog:",
     "tailwindcss": "catalog:",
     "tailwindcss-primeui": "catalog:",
     "tsx": "catalog:",