chore(deps): add HTML sanitizers for Geschichten rich-text body

Adds OWASP Java HTML Sanitizer on the backend and DOMPurify on the frontend.
Together with Tiptap on the writer side they form a defense-in-depth chain
against XSS in the new Geschichte body field (issue #381).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/package.json

@@ -25,6 +25,7 @@
 		"@tiptap/extension-mention": "3.22.5",
 		"@tiptap/starter-kit": "3.22.5",
 		"diff": "^8.0.3",
+		"dompurify": "^3.4.2",
 		"openapi-fetch": "^0.13.5",
 		"pdfjs-dist": "^5.5.207"
 	},
@@ -41,6 +42,7 @@
 		"@tailwindcss/typography": "^0.5.19",
 		"@tailwindcss/vite": "^4.1.17",
 		"@types/diff": "^7.0.2",
+		"@types/dompurify": "^3.0.5",
 		"@types/node": "^24",
 		"@vitest/browser-playwright": "^4.0.10",
 		"@vitest/coverage-v8": "^4.1.0",