fix: prevent age verification dialog flicker on page load

Initialize isOpen as false and only open in onMount if not yet verified,
instead of opening immediately and closing after localStorage check.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/lib/components/age-verification-dialog/age-verification-dialog.svelte

@@ -13,7 +13,7 @@
 
   const AGE_VERIFICATION_KEY = "age-verified";
 
-  let isOpen = true;
+  let isOpen = $state(false);
 
   function handleAgeConfirmation() {
     localStorage.setItem(AGE_VERIFICATION_KEY, "true");
@@ -21,9 +21,8 @@
   }
 
   onMount(() => {
-    const storedVerification = localStorage.getItem(AGE_VERIFICATION_KEY);
-    if (storedVerification === "true") {
-      isOpen = false;
+    if (localStorage.getItem(AGE_VERIFICATION_KEY) !== "true") {
+      isOpen = true;
     }
   });
 </script>