[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)

.stylelintrc.json

@@ -0,0 +1,73 @@
+{
+  "extends": [],
+  "overrides": [
+    {
+      "files": ["*.vue", "**/*.vue"],
+      "customSyntax": "postcss-html"
+    }
+  ],
+  "rules": {
+    "import-notation": "url",
+    "font-family-no-missing-generic-family-keyword": true,
+    "declaration-block-no-redundant-longhand-properties": true,
+    "declaration-property-value-no-unknown": [
+      true,
+      {
+        "ignoreProperties": {
+          "speak": ["none"],
+          "app-region": ["drag", "no-drag"]
+        }
+      }
+    ],
+    "color-function-notation": "modern",
+    "shorthand-property-no-redundant-values": true,
+    "selector-pseudo-element-colon-notation": "double",
+    "no-duplicate-selectors": true,
+    "font-weight-notation": "numeric",
+    "length-zero-no-unit": true,
+    "color-no-invalid-hex": true,
+    "number-max-precision": 4,
+    "property-no-vendor-prefix": true,
+    "value-no-vendor-prefix": true,
+    "selector-no-vendor-prefix": true,
+    "media-feature-name-no-vendor-prefix": true,
+    "selector-max-universal": 1,
+    "selector-max-type": 2,
+    "declaration-block-no-duplicate-properties": true,
+    "block-no-empty": true,
+    "no-descending-specificity": true,
+    "no-duplicate-at-import-rules": true,
+    "at-rule-no-unknown": [
+      true,
+      {
+        "ignoreAtRules": [
+          "tailwind",
+          "apply",
+          "layer",
+          "config",
+          "theme",
+          "reference",
+          "plugin",
+          "custom-variant",
+          "utility"
+        ]
+      }
+    ],
+    "function-no-unknown": [
+      true,
+      {
+        "ignoreFunctions": [
+          "theme"
+        ]
+      }
+    ]
+  },
+  "ignoreFiles": [
+    "node_modules/**",
+    "dist/**",
+    "playwright-report/**",
+    "public/**",
+    "src/lib/litegraph/**"
+  ],
+  "files": ["**/*.css", "**/*.vue"]
+}