fix(desktop): error response improvements — content, retry, model selector

P1: onError callback now sets content to error message instead of empty string.
    Previously API errors (404/429) produced empty assistant messages with only
    a visual error badge — now the error text is persisted in message content.

P3: Retry button now re-sends the preceding user message via sendToGateway
    instead of copying to input. Works for both virtualized and non-virtualized
    message lists. Removed unused setInput prop from MessageBubble.

Also hides model selector in Tauri runtime (SaaS token pool routes models).

desktop/src/store/chat/streamStore.ts

@@ -502,10 +502,17 @@ export const useStreamStore = create<StreamState>()(
             }
           },
           onError: (error: string) => {
+            // Flush any remaining buffered deltas before erroring
+            if (flushTimer !== null) {
+              clearTimeout(flushTimer);
+              flushTimer = null;
+            }
+            flushBuffers();
+
             _chat?.updateMessages(msgs =>
               msgs.map(m =>
                 m.id === assistantId
-                  ? { ...m, content: '', streaming: false, error }
+                  ? { ...m, content: error, streaming: false, error }
                   : m.role === 'user' && m.optimistic && m.timestamp.getTime() >= streamStartTime
                     ? { ...m, optimistic: false }
                     : m