fix unreachable error branch in fetchConversation causing silent failures

openapi-fetch returns either `{ data }` or `{ error }`, never both. The previous guard checked `!data` first, so HTTP errors always hit that branch and returned early — the `error` toast was unreachable. Failed conversation loads now surface to the user instead of failing silently.

Co-Authored-By: John Chilton <jmchilton@gmail.com>

client/src/components/GalaxyAI.vue

@@ -281,12 +281,13 @@ async function fetchConversation(exchangeId: string) {
         },
     });
 
-    if (!fullConversation || fullConversation.length === 0) {
-        return;
-    } else if (error) {
+    if (error) {
         Toast.error(errorMessageAsString(error, "Failed to load conversation."), "Error loading conversation");
         return;
     }
+    if (!fullConversation || fullConversation.length === 0) {
+        return;
+    }
 
     messages.value = fullConversation.map((msg: any, index: number) => {
         const message: ChatMessage = {