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/useSessionLifecycle.ts

@@ -102,30 +102,25 @@ export function useSessionLifecycle(opts: UseSessionLifecycleOptions) {
         return patchUiState({ status: 'ready' })
       }
 
+      const info = r.info ?? null
+
       resetSession()
       setSessionStartedAt(Date.now())
-      // Python's `session.create` returns instantly with partial info (no `version`
-      // field); the `session.info` event will flip status to 'ready' once the
-      // agent is fully built (~1s later). Until then prompt.submit will block
-      // server-side on `_wait_agent`.
+
+      // session.create returns instantly with partial info (no `version`);
+      // the `session.info` event flips status to 'ready' once the agent is live.
       patchUiState({
-        info: r.info ?? null,
+        info,
         sid: r.session_id,
-        status: r.info?.version ? 'ready' : 'starting agent…',
-        usage: usageFrom(r.info ?? null)
+        status: info?.version ? 'ready' : 'starting agent…',
+        usage: usageFrom(info)
       })
 
-      if (r.info) {
-        setHistoryItems([introMsg(r.info)])
-      }
+      if (info) setHistoryItems([introMsg(info)])
 
-      if (r.info?.credential_warning) {
-        sys(`warning: ${r.info.credential_warning}`)
-      }
+      if (info?.credential_warning) sys(`warning: ${info.credential_warning}`)
 
-      if (msg) {
-        sys(msg)
-      }
+      if (msg) sys(msg)
     },
     [closeSession, colsRef, resetSession, rpc, setHistoryItems, setSessionStartedAt, sys]
   )