feat: migrate to react-markdown for robust markdown rendering

- Replace custom markdown parser with react-markdown + remark-gfm
- Fix document link navigation (./ and ../ references)
- Simplify doc viewing flow (direct markdown content instead of parsed structure)
- Update electron main process to only use api folder for docs
- Add blueprint loading from docs/blueprint.md dynamically
- Fix sidebar file selection path matching
- Update preload scripts for new API structure

package.json

@@ -21,6 +21,8 @@
     "lucide-react": "^0.511.0",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
+    "react-markdown": "^10.1.0",
+    "remark-gfm": "^4.0.1",
     "tailwind-merge": "^3.5.0",
     "three": "^0.183.2",
     "zustand": "^5.0.11"