Stop loading the whole version twice on every page load

The version re-validation effect could not tell "the versions fetch has not
landed yet" from "this source has no versions", because both look like an
empty array. So a versionId restored from localStorage was cleared on the
first render and set straight back when the fetch returned: 29 -> '' -> 29.

Forecast's load effect is keyed on [versionId, sourceId], so that round
trip ran initViewer twice. Two /agg requests, two full aggregations in
Postgres, two Arrow payloads -- on version 29 that is the 285k-row
aggregate computed twice at ~17s each, which is most of the "it hangs
before it displays" and the Postgres process sitting at the top of htop.

sourcesLoaded already guarded the identical effect for sources; versions
just never got the same treatment. Cleared when the source changes, since
the list belongs to the previous source until the new fetch lands.

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

ui/src/App.jsx

@@ -13,6 +13,7 @@ export default function App() {
   const [sourcesLoaded, setSourcesLoaded] = useState(false)
   const [sourceId,  setSourceId]  = useState(() => localStorage.getItem('pf_sourceId')  || '')
   const [versions,  setVersions]  = useState([])
+  const [versionsLoaded, setVersionsLoaded] = useState(false)
   const [versionId, setVersionId] = useState(() => localStorage.getItem('pf_versionId') || '')
 
   useEffect(() => { localStorage.setItem('pf_view', view) }, [view])
@@ -30,10 +31,11 @@ export default function App() {
 
   const refreshVersions = useCallback(async (sid) => {
     const id = sid ?? sourceId
-    if (!id) { setVersions([]); return [] }
+    if (!id) { setVersions([]); setVersionsLoaded(true); return [] }
     const data = await fetch(`/api/sources/${id}/versions`).then(r => r.json())
     const list = Array.isArray(data) ? data : []
     setVersions(list)
+    setVersionsLoaded(true)
     return list
   }, [sourceId])
 
@@ -52,18 +54,26 @@ export default function App() {
   }, [sources, sourcesLoaded, sourceId])
 
   useEffect(() => {
-    if (!sourceId) { setVersions([]); setVersionId(''); return }
+    if (!sourceId) { setVersions([]); setVersionId(''); setVersionsLoaded(true); return }
+    // The list belongs to the previous source until the fetch lands.
+    setVersionsLoaded(false)
     refreshVersions(sourceId)
   }, [sourceId])
 
   // Same reasoning as sources: a deleted version must not stay selected.
+  //
+  // versionsLoaded matters more than it looks: without it, the empty initial
+  // state reads as "this source has no versions", so a versionId restored from
+  // localStorage is cleared and then set straight back when the fetch lands.
+  // Forecast's load effect is keyed on that id, so the round trip made every
+  // page load fetch and aggregate the whole version twice.
   useEffect(() => {
-    if (!sourceId) return
+    if (!sourceId || !versionsLoaded) return
     if (versions.length === 0) { setVersionId(''); return }
     if (!versionId || !versions.some(v => String(v.id) === String(versionId))) {
       setVersionId(String(versions[0].id))
     }
-  }, [versions, sourceId, versionId])
+  }, [versions, versionsLoaded, sourceId, versionId])
 
   const ctx = {
     sources, sourceId, setSourceId,