Fetch col_meta in the member-list effect instead of reading a ref

The effect decided which dim_groups to load from colMetaRef.current, but
that ref is filled inside initViewer, which is async. The effect ran first,
saw an empty array, found no groups, and fetched nothing -- and a ref
changing does not re-run an effect, so it never recovered. dimMembers stayed
{} for the whole session.

Everything therefore fell back to the source lookup, which is precisely the
query pf.dim_member exists to replace: XCP06500G18B112 has two attribute
combinations across history, so DISTINCT ... LIMIT 2 returned two rows and
the route answered null. The member row had the answer all along, picked as
the most recent by order date.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/src/views/Forecast.jsx

@@ -669,7 +669,16 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
     if (!sourceId) { setDimMembers({}); return }
     let cancelled = false
     ;(async () => {
-      const meta   = colMetaRef.current
+      // col_meta is fetched here rather than read from colMetaRef: that ref is
+      // filled inside initViewer, which is async, so this effect ran first, saw an
+      // empty array, found no groups and never fetched a list -- and a ref changing
+      // does not re-run an effect, so it never recovered. It is a small query and
+      // the browser will serve it from cache anyway.
+      let meta = []
+      try {
+        meta = await fetch(`/api/sources/${sourceId}/cols`).then(r => r.ok ? r.json() : [])
+      } catch { return }
+      if (cancelled) return
       const groups = [...new Set(meta.filter(c => c.dim_group && c.is_key).map(c => c.dim_group))]
       const loaded = {}
       for (const g of groups) {