Retry the strategy list and make collapse obvious

- The strategy/preset lists were loaded once with failures swallowed, so a
  single failed first request left the strategy dropdown empty forever. They
  are now retried every 3s until populated.
- Clearer collapse affordance: hover highlight, a hide/show hint, and a
  tooltip; Backtest and Parameter sweep start collapsed so the strategy
  selector and status are immediately visible.
- Adds an integration test that the strategy dropdown is populated and the
  panel actually collapses.

frontend/src/components/SweepPanel.vue

@@ -92,7 +92,7 @@ async function run() {
 </script>
 
 <template>
-  <CollapsiblePanel title="Parameter sweep" storage-key="sweep" :summary="summary">
+  <CollapsiblePanel title="Parameter sweep" storage-key="sweep" :summary="summary" :default-open="false">
     <div class="space-y-3">
     <div class="grid gap-2 sm:grid-cols-4">
       <label class="space-y-1">