Add ValidScopedCSSClass theme check

Add a new theme check that warns when a CSS class used in an HTML
class attribute may be defined outside the scope of the current file,
helping catch misplaced styles and dependency issues.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aswamy

.changeset/rude-poems-yell.md

@@ -0,0 +1,5 @@
+---
+'@shopify/theme-graph': patch
+---
+
+[internal] Moving @shopify/theme-check-node to being a dev-dependency of the package

.changeset/small-items-marry.md

@@ -0,0 +1,14 @@
+---
+'@shopify/theme-language-server-common': minor
+'@shopify/theme-check-common': minor
+'@shopify/theme-check-node': minor
+---
+
+Introduce new theme-check to detect invalid CSS class usage in HTML
+
+`ValidScopedCSSClass` detects invalid CSS classes in HTML if it deems them being out of scope.
+A CSS class is in-scope if it matches any of the following:
+- The CSS class is declared in `assets/*.css` file
+- The CSS class is declared in the file's `stylesheet` tag
+- The CSS class is declared in a *direct* ancestor's `stylesheet` tag
+- The CSS class is declared in a snippet file's `stylesheet` tag that is rendered by this file or its direct ancestor (recursive)

packages/theme-check-common/package.json

@@ -32,11 +32,15 @@
     "line-column": "^1.0.2",
     "lodash": "^4.17.23",
     "minimatch": "^10.2.1",
+    "postcss": "^8.5.9",
+    "postcss-safe-parser": "^7.0.1",
+    "postcss-selector-parser": "^7.1.1",
     "vscode-json-languageservice": "^5.3.10",
     "vscode-uri": "^3.0.7"
   },
   "devDependencies": {
     "@types/line-column": "^1.0.0",
-    "@types/lodash": "^4.17.20"
+    "@types/lodash": "^4.17.20",
+    "@types/postcss-safe-parser": "^5.0.4"
   }
 }

packages/theme-check-common/src/checks/index.ts

@@ -52,6 +52,7 @@ import { UnusedDocParam } from './unused-doc-param';
 import { ValidContentForArguments } from './valid-content-for-arguments';
 import { ValidContentForArgumentTypes } from './valid-content-for-argument-types';
 import { ValidBlockTarget } from './valid-block-target';
+import { ValidScopedCSSClass } from './valid-scoped-css-class';
 import { ValidHTMLTranslation } from './valid-html-translation';
 import { ValidJSON } from './valid-json';
 import { ValidDocParamTypes } from './valid-doc-param-types';
@@ -123,6 +124,7 @@ export const allChecks: (LiquidCheckDefinition | JSONCheckDefinition)[] = [
   ValidHTMLTranslation,
   ValidContentForArguments,
   ValidContentForArgumentTypes,
+  ValidScopedCSSClass,
   ValidJSON,
   ValidDocParamTypes,
   ValidLocalBlocks,