fix: replace gray-matter with direct yaml.load for js-yaml 4.x compatibility (#26555)

gray-matter uses yaml.safeLoad() which was removed in js-yaml 4.x, causing 500
errors on app store pages after the js-yaml 4.1.1 update (CWE-1321 fix)

- Add parseFrontmatter function using yaml.load with JSON_SCHEMA
- Add type guard for safe type narrowing
- Add unit tests for frontmatter parsing and security
- Remove gray-matter dependency

apps/web/package.json

@@ -91,7 +91,6 @@
     "classnames": "2.3.2",
     "dompurify": "3.3.1",
     "entities": "4.5.0",
-    "gray-matter": "4.0.3",
     "handlebars": "4.7.7",
     "i18next": "23.2.3",
     "ical.js": "1.5.0",