Fix duplicate gem icon: defer assistant message creation until first token

Instead of pre-creating an empty placeholder assistant message when
streaming starts (which races with GeminiTypingIndicator rendering),
wait until the first SSE token arrives to create the message. This
guarantees only one row is ever visible: either the typing indicator
(before tokens) or the GeminiMessage (after first token).

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

frontend/components/gemini/GeminiChat.tsx

@@ -193,23 +193,9 @@ export function GeminiChat() {
       );
     }
 
-    // Create a placeholder AI message for streaming
     const aiMsgId = `msg-${Date.now()}-ai`;
-    const aiMsg: Message = {
-      id: aiMsgId,
-      role: "assistant",
-      content: "",
-      timestamp: new Date(),
-    };
-
     const streamConvId = convId;
 
-    setConversations((prev) =>
-      prev.map((c) =>
-        c.id === streamConvId ? { ...c, messages: [...c.messages, aiMsg] } : c
-      )
-    );
-
     abortRef.current = streamChat(
       text,
       streamConvId,
@@ -220,6 +206,23 @@ export function GeminiChat() {
           setConversations((prev) =>
             prev.map((c) => {
               if (c.id !== streamConvId) return c;
+              const exists = c.messages.some((m) => m.id === aiMsgId);
+              if (!exists) {
+                // First token: create the assistant message
+                return {
+                  ...c,
+                  messages: [
+                    ...c.messages,
+                    {
+                      id: aiMsgId,
+                      role: "assistant" as const,
+                      content: event.content!,
+                      timestamp: new Date(),
+                    },
+                  ],
+                };
+              }
+              // Subsequent tokens: append to existing message
               return {
                 ...c,
                 messages: c.messages.map((m) =>
@@ -231,7 +234,6 @@ export function GeminiChat() {
             })
           );
         }
-        // tool_start / tool_end events could be handled here for UI indicators
       },
       () => {
         // on error