feat(tui): queue pre-session input, auto-flush when session lands

The TUI is fully interactive from the first frame but `session.create`
(agent + tools + MCP) takes ~2s. Plain-text messages typed before the
session is live used to fail with "session not ready yet"; slash and
shell commands worked but agent prompts were dropped.

Now:
- `dispatchSubmission` enqueues plain text when `sid` is null (slash/shell
  still short-circuit first)
- `useMainApp` tracks sid transitions and kicks off one `sendQueued()`
  when the session first becomes ready; subsequent queued messages drain
  on `message.complete` as before
- Fixed pre-existing double-Enter bug that dequeued without sid check

User flow: type `hello` → shows in `queuedDisplay` preview → 2s later
agent wakes → message auto-sends → reply streams. Zero wasted input.

ui-tui/src/app/useMainApp.ts

@@ -372,6 +372,28 @@ export function useMainApp(gw: GatewayClient) {
     sys
   })
 
+  // Flush any pre-session queued input once the session lands.
+  // Message.complete already drains subsequent items; this only kicks off the first.
+  const prevSidRef = useRef<null | string>(null)
+  useEffect(() => {
+    const prev = prevSidRef.current
+    prevSidRef.current = ui.sid
+
+    if (prev !== null || !ui.sid || ui.busy) {
+      return
+    }
+
+    if (composerRefs.queueEditRef.current !== null) {
+      return
+    }
+
+    const next = composerActions.dequeue()
+
+    if (next) {
+      sendQueued(next)
+    }
+  }, [ui.sid, ui.busy, composerActions, composerRefs, sendQueued])
+
   const { pagerPageSize } = useInputHandlers({
     actions: {
       answerClarify,