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)

src/hooks/useApi.js

@@ -134,3 +134,15 @@ export async function fetchDrawLocation(state, race, sex, year, model) {
 
   return apiFetch(`/draws?${params}`)
 }
+
+/** GET /estimates/{leaid}/draws?model=X&year=Y — fetch posterior draws for a district */
+export async function fetchDistrictDraws(leaid, options = {}) {
+  const params = new URLSearchParams()
+  if (options.model) params.set('model', options.model)
+  if (options.year) params.set('year', options.year)
+  if (options.race) params.set('race', options.race)
+  if (options.sex) params.set('sex', options.sex)
+
+  const qs = params.toString()
+  return apiFetch(`/estimates/${leaid}/draws${qs ? `?${qs}` : ''}`)
+}