feat(geschichten): frontend foundation — canBlogWrite, sanitize util, nav, i18n

- Derives canBlogWrite in +layout.server.ts the same way as canAnnotate.
- Adds Geschichten link to AppNav (desktop + mobile, between Stammbaum and Admin).
- Adds error_geschichte_not_found mapping to errors.ts and translation keys
  for the Geschichten index, detail, editor, and confirmation copy in
  de/en/es.
- Adds isomorphic-dompurify-backed safeHtml() helper with allow-list
  matching the backend OWASP policy (p/br/strong/em/h2/h3/ul/ol/li),
  plus Vitest spec.
- Updates legacy spec test data so the new required canBlogWrite layout
  prop type-checks.

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

frontend/package.json

@@ -25,7 +25,7 @@
 		"@tiptap/extension-mention": "3.22.5",
 		"@tiptap/starter-kit": "3.22.5",
 		"diff": "^8.0.3",
-		"dompurify": "^3.4.2",
+		"isomorphic-dompurify": "^3.12.0",
 		"openapi-fetch": "^0.13.5",
 		"pdfjs-dist": "^5.5.207"
 	},
@@ -42,7 +42,6 @@
 		"@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",