fix: prevent duplicate AI message when switching conversations during stream

Two bugs caused the same AI response to appear twice:

1. handleSelectConversation would fetch conversation history even while
   a stream was in progress (isLoading). If the fetch resolved mid-stream,
   fetchConversation overwrote the conversation messages unconditionally,
   writing the server-persisted AI message (with a real UUID) alongside the
   still-streaming client-side message (with a temp ID), producing a duplicate.
   Fix: guard the fetch with !isLoading, and add a second check inside the
   .then callback so setConversations only writes history if the messages
   array is still empty (protects against slow-resolving network requests).

2. onDone in streamChat could theoretically be called a second time if the
   ReadableStream completed without a "done" SSE event. Fix: call
   reader.cancel() immediately after onDone so the while loop exits cleanly.

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

frontend/components/gemini/GeminiChat.tsx

@@ -185,9 +185,9 @@ export function GeminiChat() {
       setActiveConvId(id);
       setInputValue("");
 
-      // Load messages if not yet loaded
+      // Load messages if not yet loaded and no stream is in progress
       const conv = conversations.find((c) => c.id === id);
-      if (conv && conv.messages.length === 0) {
+      if (conv && conv.messages.length === 0 && !isLoading) {
         fetchConversation(id)
           .then((detail) => {
             const msgs: Message[] = detail.messages.map((m) => ({
@@ -196,8 +196,14 @@ export function GeminiChat() {
               content: m.content,
               timestamp: new Date(m.created_at),
             }));
+            // Only write history if the conversation still has no messages
+            // (guards against a race where streaming already populated it)
             setConversations((prev) =>
-              prev.map((c) => (c.id === id ? { ...c, messages: msgs } : c))
+              prev.map((c) =>
+                c.id === id && c.messages.length === 0
+                  ? { ...c, messages: msgs }
+                  : c
+              )
             );
           })
           .catch(() => {
@@ -205,7 +211,7 @@ export function GeminiChat() {
           });
       }
     },
-    [conversations]
+    [conversations, isLoading]
   );
 
   const handleRenameConversation = useCallback(

frontend/lib/api.ts

@@ -209,6 +209,7 @@ export function streamChat(
             onEvent(event);
             if (event.type === "done") {
               onDone();
+              reader.cancel();
               return;
             }
           } catch {
@@ -216,6 +217,7 @@ export function streamChat(
           }
         }
       }
+      // Only call onDone here if the stream ended without a "done" event
       onDone();
     })
     .catch((err) => {