perf(tui): paint banner on first frame, don't wait on session.create

Previously `historyItems` was seeded empty and the intro (with Banner +
SessionPanel) was only pushed after Python's `session.create` returned —
~1.8s of agent + tools + MCP init with nothing on screen. Base CLI feels
instant because it prints the banner as its first action.

Seed `historyItems` with an info-less intro on mount. `appLayout` now
renders the Banner unconditionally for `kind === 'intro'` and gates only
the SessionPanel on `info` being present. Gateway.ready swaps the skin
(~200ms) and session.info fills in the panel when the agent is ready.

Net: first usable frame drops from ~2s to ~300ms (node + module graph +
React mount). No behavior change — intro message is replaced in place
by `introMsg(info)` when `newSession()` / `resumeById()` resolve.

ui-tui/src/components/appLayout.tsx

@@ -33,11 +33,11 @@ const TranscriptPane = memo(function TranscriptPane({
 
           {visibleHistory.map(row => (
             <Box flexDirection="column" key={row.key} ref={transcript.virtualHistory.measureRef(row.key)}>
-              {row.msg.kind === 'intro' && row.msg.info ? (
+              {row.msg.kind === 'intro' ? (
                 <Box flexDirection="column" paddingTop={1}>
                   <Banner t={ui.theme} />
 
-                  <SessionPanel info={row.msg.info} sid={ui.sid} t={ui.theme} />
+                  {row.msg.info && <SessionPanel info={row.msg.info} sid={ui.sid} t={ui.theme} />}
                 </Box>
               ) : row.msg.kind === 'panel' && row.msg.panelData ? (
                 <Panel sections={row.msg.panelData.sections} t={ui.theme} title={row.msg.panelData.title} />