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/interface/components/store/ThemeModal.svelte

@@ -234,7 +234,7 @@
 
   $effect(() => {
 
-    if (displayTheme) {
+    if (displayTheme && modalElement) {
 
       animate(