Add D3 and new ridgeline chart matching R aesthetic

- Install d3@7
- Add fetchDistrictDraws API function (for future use with raw draws)
- Create RateDensityRidgeline component using D3.js
  - Density ridges showing posterior distributions per race group
  - Diamond markers for observed rates (matching R design)
  - Dropdown to switch between model specifications
  - Matches Civilytics color palette (navy fill, danger diamonds)

package.json

@@ -25,5 +25,8 @@
     "react-dom": "^19.2.8",
     "vite": "^8.2.1"
   },
-  "type": "module"
+  "type": "module",
+  "dependencies": {
+    "d3": "^7.9.0"
+  }
 }