refactor(tui): clean up touched files — DRY, KISS, functional

Python (tui_gateway/server.py):
- hoist `_wait_agent` next to `_sess` so `_sess` no longer forward-refs
- simplify `_wait_agent`: `ready.wait()` already returns True when set,
  no separate `.is_set()` check, collapse two returns into one expr
- factor `_sess_nowait` for handlers that don't need the agent (currently
  `terminal.resize` + `input.detect_drop`) — DRY up the duplicated
  `_sessions.get` + "session not found" dance
- inline `session = _sessions[sid]` in the session.create build thread so
  agent/worker writes don't re-look-up the dict each time
- rename inline `ready_event` → `ready` (it's never ambiguous)

TS:
- `useSessionLifecycle.newSession`: hoist `r.info ?? null` into `info`
  so it's one lookup, drop ceremonial `{ … }` blocks around single-line
  bodies
- `createGatewayEventHandler.session.info`: wrap the case in a block,
  hoist `ev.payload` into `info`, tighten comments
- `useMainApp` flush effect: collapse two guard returns into one
- `bootBanner.ts`: lift `TAGLINE` + `FALLBACK` to module constants, make
  `GRADIENT` readonly, one-liner return via template literal
- `theme.ts`: group `selectionBg` inside the status* block (it's a UI
  surface bg, same family), trim the comment

ui-tui/src/app/useMainApp.ts

@@ -379,26 +379,19 @@ 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.
+  // Flush any pre-session queued input the moment the session lands.
+  // `message.complete` drains the rest; this just kicks off the first send.
   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) {
+    if (prev !== null || !ui.sid || ui.busy || composerRefs.queueEditRef.current !== null) {
       return
     }
 
     const next = composerActions.dequeue()
-
-    if (next) {
-      sendQueued(next)
-    }
+    if (next) sendQueued(next)
   }, [ui.sid, ui.busy, composerActions, composerRefs, sendQueued])
 
   const { pagerPageSize } = useInputHandlers({