fix: Tabellen-Rendering und Prose-Styling verbessert

- remark-gfm hinzugefuegt fuer GFM-Tabellen-Support
- Tabellen-CSS: Header, Hover, Spacing, responsive overflow
- Prose H1-Styling fuer Guide-Titel aus Markdown
- Horizontale Linien, Strong-Tags, Listen in Tabellen
- Alle Guides: Tabellen werden jetzt als HTML-<table> gerendert

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

app/package.json

@@ -17,6 +17,7 @@
     "react-dom": "19.2.4",
     "reading-time": "^1.5.0",
     "remark": "^15.0.1",
+    "remark-gfm": "^4.0.1",
     "remark-html": "^16.0.1"
   },
   "devDependencies": {