fix: hide typing indicator once assistant message appears during streaming

The "two diamond" bug was caused by GeminiTypingIndicator (which has its
own gem icon) rendering simultaneously with GeminiMessage (also has a gem
icon) once the first token arrived. Now the typing indicator only shows
while isLoading is true AND no assistant message exists yet (last message
is not role "assistant").

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

frontend/components/gemini/GeminiChat.tsx

@@ -367,7 +367,9 @@ export function GeminiChat() {
                   />
                 );
               })}
-              {isLoading && <GeminiTypingIndicator />}
+              {isLoading && messages.length > 0 && messages[messages.length - 1].role !== "assistant" && (
+                <GeminiTypingIndicator />
+              )}
               <div ref={messagesEndRef} />
             </div>
           )}