fix: preserve split UTF-8 characters in streaming responses

- Decode API response chunks with persistent streaming state
- Reuse streaming decoder when inspecting chat backend events

web/src/lib/api/base.ts

@@ -46,12 +46,14 @@ export const api = {
     const reader = response.body?.getReader();
     if (!reader) throw new Error('Response body is null');
 
+    // Decode in streaming mode: a multi-byte UTF-8 rune split across network
+    // chunks is otherwise decoded as two halves and corrupted into U+FFFD.
     const decoder = new TextDecoder();
     while (true) {
       const { done, value } = await reader.read();
-      yield decoder.decode(value);
-
       if (done) break;
+
+      yield decoder.decode(value, { stream: true });
     }
   }
 };

web/src/routes/chat/+server.ts

@@ -116,10 +116,13 @@ export const POST: RequestHandler = async ({ request }) => {
       throw new Error('No response from fabric backend');
     }
 
-    // Create a TransformStream to inspect the data without modifying it
+    // Create a TransformStream to inspect the data without modifying it.
+    // The decoder is persistent and streaming: a multi-byte UTF-8 rune split
+    // across chunks is otherwise logged as two halves corrupted into U+FFFD.
+    const decoder = new TextDecoder();
     const transformStream = new TransformStream({
       transform(chunk, controller) {
-        const text = new TextDecoder().decode(chunk);
+        const text = decoder.decode(chunk, { stream: true });
         if (text.startsWith('data: ')) {
           try {
             const data = JSON.parse(text.slice(6));