feat(web): add Tiptap WYSIWYG KnowledgeEditor component

Install @tiptap/react + extensions (starter-kit, link, table,
code-block-lowlight, placeholder) and build KnowledgeEditor component
with full toolbar: headings, bold, italic, strikethrough, lists,
blockquote, code block with syntax highlighting, links, tables,
and horizontal rules. All styles use CSS variables for theme compat.

Implements TW-EDT-001.

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

apps/web/package.json

@@ -22,12 +22,23 @@
     "@mosaic/shared": "workspace:*",
     "@mosaic/ui": "workspace:*",
     "@tanstack/react-query": "^5.90.20",
+    "@tiptap/extension-code-block-lowlight": "^3.20.0",
+    "@tiptap/extension-link": "^3.20.0",
+    "@tiptap/extension-placeholder": "^3.20.0",
+    "@tiptap/extension-table": "^3.20.0",
+    "@tiptap/extension-table-cell": "^3.20.0",
+    "@tiptap/extension-table-header": "^3.20.0",
+    "@tiptap/extension-table-row": "^3.20.0",
+    "@tiptap/pm": "^3.20.0",
+    "@tiptap/react": "^3.20.0",
+    "@tiptap/starter-kit": "^3.20.0",
     "@types/dompurify": "^3.2.0",
     "@xyflow/react": "^12.5.3",
     "better-auth": "^1.4.17",
     "date-fns": "^4.1.0",
     "dompurify": "^3.3.1",
     "elkjs": "^0.9.3",
+    "lowlight": "^3.3.0",
     "lucide-react": "^0.563.0",
     "mermaid": "^11.4.1",
     "next": "^16.1.6",