Wrap chunk content in a Chunk type

Preparing to include additional information with each chunk (e.g. token
count)

pkg/tui/views/chat/update.go

@@ -90,20 +90,19 @@ func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
 	case msgResponseChunk:
 		cmds = append(cmds, m.waitForResponseChunk()) // wait for the next chunk
 
-		chunk := string(msg)
-		if chunk == "" {
+		if msg.Content == "" {
 			break
 		}
 
 		last := len(m.messages) - 1
 		if last >= 0 && m.messages[last].Role.IsAssistant() {
 			// append chunk to existing message
-			m.setMessageContents(last, m.messages[last].Content+chunk)
+			m.setMessageContents(last, m.messages[last].Content+msg.Content)
 		} else {
 			// use chunk in new message
 			m.addMessage(models.Message{
 				Role:    models.MessageRoleAssistant,
-				Content: chunk,
+				Content: msg.Content,
 			})
 		}
 		m.updateContent()