fix: theme store stuck on loading skeleton after fetch failures

Harden theme list fetching with normalized API responses, timeouts, retries,
and a visible error state so the store no longer stays blank when messaging
or payloads fail (common after extension updates without a SEQTA tab reload).

src/seqta/ui/renderStore.ts

@@ -15,6 +15,9 @@ export function renderStore() {
     throw new Error("Container not found");
   }
 
+  // Avoid stacking multiple store roots if opened repeatedly without close.
+  document.getElementById("store")?.remove();
+
   const child = document.createElement("div");
   child.id = "store";
   container!.appendChild(child);