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/BacktestPanel.vue

@@ -37,7 +37,7 @@ async function run(payload) {
 </script>
 
 <template>
-  <CollapsiblePanel title="Backtest" storage-key="backtest" :summary="summary">
+  <CollapsiblePanel title="Backtest" storage-key="backtest" :summary="summary" :default-open="false">
     <div class="space-y-3">
       <StrategyForm
       :strategies="strategies"

frontend/src/components/CollapsiblePanel.vue

@@ -39,8 +39,9 @@ watch(open, (value) => {
   <section class="card">
     <button
       type="button"
-      class="flex w-full items-center justify-between gap-3 text-left"
+      class="-m-2 flex w-full items-center justify-between gap-3 rounded-lg p-2 text-left transition hover:bg-slate-700/40"
       :aria-expanded="open"
+      :title="open ? 'Collapse' : 'Expand'"
       @click="open = !open"
     >
       <span class="flex items-center gap-2 font-semibold">
@@ -54,6 +55,7 @@ watch(open, (value) => {
           <path d="M6 3.5 11 8l-5 4.5z" />
         </svg>
         {{ title }}
+        <span class="text-xs font-normal text-slate-500">{{ open ? 'hide' : 'show' }}</span>
       </span>
       <span v-if="!open && summary" class="truncate text-xs font-normal text-slate-400">
         {{ summary }}

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">