fix(tui): keep the newline above the streaming assistant text

Finalized assistant messages rendered the thinking/tools trail inside
MessageLine with marginBottom=1 before the response body — giving a
clean blank line above the text. The streaming path rendered the
progress ToolTrail and the streaming MessageLine as two separate
siblings with no margin between, so the in-progress response butted
right up against the thinking panel. That's the "newline appears
after it's done" jank.

Wrap the streaming MessageLine in a Box with marginTop=1 whenever the
progress area is visible above it. Same spacing as the finalized
version, continuous through the handoff.

ui-tui/src/components/appLayout.tsx

@@ -73,14 +73,16 @@ const TranscriptPane = memo(function TranscriptPane({
           )}
 
           {progress.showStreamingArea && (
-            <MessageLine
-              cols={composer.cols}
-              compact={ui.compact}
-              detailsMode={ui.detailsMode}
-              isStreaming
-              msg={{ role: 'assistant', text: progress.streaming }}
-              t={ui.theme}
-            />
+            <Box flexDirection="column" marginTop={progress.showProgressArea ? 1 : 0}>
+              <MessageLine
+                cols={composer.cols}
+                compact={ui.compact}
+                detailsMode={ui.detailsMode}
+                isStreaming
+                msg={{ role: 'assistant', text: progress.streaming }}
+                t={ui.theme}
+              />
+            </Box>
           )}
         </Box>
       </ScrollBox>