fix: clear stale drawsByGroup when a model switch starts or fails

useDrawDistribution never cleared drawsByGroup on a new fetch or on
error, only status. A chart that varies `model` across renders (Chart
3's dropdown) could switch from a model that had succeeded to one
that then failed, and keep rendering the *previous* model's real
posterior draws — keyed by the same RACE_SEX strings — under the
newly-selected model's summary stats, with ApproxNote visible
suggesting (wrongly) that the fallback approximation was in use.

Clear drawsByGroup to null both when a new fetch starts and in the
catch branch, so a failed model switch never mixes draws from two
different models.

src/hooks/useDrawDistribution.js

@@ -63,6 +63,14 @@ export function useDrawDistribution({ leaid, state, model, year, groups }) {
     if (!leaid || !state || !model || !year || !groups?.length) return
     let cancelled = false
     setStatus('loading')
+    // Clear any previous model/district's draws immediately, not just on
+    // success/failure below. Without this, a chart that varies `model`
+    // across renders (e.g. RateDensityRidgeline's dropdown) would keep
+    // rendering the *previous* model's real draws — keyed by the same
+    // RACE_SEX strings — under the newly-selected model's summary stats
+    // while this fetch is in flight or if it fails, silently mixing data
+    // from two different models.
+    setDrawsByGroup(null)
 
     async function run() {
       let conn
@@ -106,7 +114,14 @@ export function useDrawDistribution({ leaid, state, model, year, groups }) {
         }
       } catch (err) {
         console.error('useDrawDistribution failed:', err)
-        if (!cancelled) setStatus('error')
+        if (!cancelled) {
+          // Belt-and-suspenders alongside the setDrawsByGroup(null) at the
+          // top of this effect: a failed fetch/query must never leave a
+          // *previous* model's real draws in place under the newly-selected
+          // model's status/summary stats.
+          setDrawsByGroup(null)
+          setStatus('error')
+        }
       } finally {
         if (conn) await conn.close()
       }