feat(web): add markdown round-trip and replace textarea with Tiptap editor

Add tiptap-markdown extension for markdown ↔ rich text conversion.
Update KnowledgeEditor to accept/emit markdown content. Replace
EntryEditor textarea with KnowledgeEditor (Tiptap WYSIWYG). Update
tests for the new editor component.

Content flow: markdown (stored) → Tiptap (editing) → markdown (saved).
The backend markdown → HTML rendering pipeline remains unchanged.

Implements TW-EDT-002.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/package.json

@@ -46,7 +46,8 @@
     "react-dom": "^19.0.0",
     "react-grid-layout": "^2.2.2",
     "recharts": "^3.7.0",
-    "socket.io-client": "^4.8.3"
+    "socket.io-client": "^4.8.3",
+    "tiptap-markdown": "^0.9.0"
   },
   "devDependencies": {
     "@mosaic/config": "workspace:*",