fix(tui): drain message queue on every busy → false transition

Previously the queue only drained inside the message.complete event
handler, so anything enqueued while a shell.exec (!sleep, !cmd) or a
failed agent turn was running would stay stuck forever — neither of
those paths emits message.complete. After Ctrl+C an interrupted
session would also orphan the queue because idle() flips busy=false
locally without going through message.complete.

Single source of truth: a useEffect that watches ui.busy. When the
session is settled (sid present, busy false, not editing a queue
item), pull one message and send it. Covers agent turn end,
interrupt, shell.exec completion, error recovery, and the original
startup hydration (first-sid case) all at once.

Dropped the now-redundant dequeue/sendQueued from
createGatewayEventHandler.message.complete and the accompanying
GatewayEventHandlerContext.composer field — the effect handles it.

ui-tui/src/app/interfaces.ts

@@ -193,11 +193,6 @@ export interface InputHandlerResult {
 }
 
 export interface GatewayEventHandlerContext {
-  composer: {
-    dequeue: () => string | undefined
-    queueEditRef: MutableRefObject<null | number>
-    sendQueued: (text: string) => void
-  }
   gateway: GatewayServices
   session: {
     STARTUP_RESUME_ID: string