fix: use findLastChunkIndex for chunk accumulation

Chunk messages from the same persona were displayed as separate lines
instead of being concatenated into a single streaming message. The root
cause: audio (TTS) and system (typing indicator) messages interleaved
between chunks, and the accumulator only checked the very last array
element. Now uses findLastChunkIndex to search backward through the
full message array, matching the logic already used for final message
replacement.

apps/web/src/hooks/useChatState.ts

@@ -175,13 +175,12 @@ export function useChatState(): UseChatStateReturn {
         const chunkColor = typeof msg.color === "string" ? msg.color : undefined;
         const chunkSeq = typeof msg.seq === "number" ? msg.seq : undefined;
         setMessages((prev) => {
-          // Find the last message from this nick that is a chunk (still streaming)
-          const lastIdx = prev.length - 1;
-          const last = lastIdx >= 0 ? prev[lastIdx] : null;
-          if (last && last.type === "chunk" && last.nick === chunkNick) {
-            // Append to existing chunk message
+          // Find the last chunk from this nick anywhere in the array (audio/system messages may interleave)
+          const chunkIdx = findLastChunkIndex(prev, chunkNick);
+          if (chunkIdx >= 0) {
+            const existing = prev[chunkIdx];
             const updated = [...prev];
-            updated[lastIdx] = { ...last, text: (last.text || "") + chunkText, seq: chunkSeq ?? last.seq };
+            updated[chunkIdx] = { ...existing, text: (existing.text || "") + chunkText, seq: chunkSeq ?? existing.seq };
             return updated;
           }
           // New streaming message