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/createGatewayEventHandler.ts

@@ -161,21 +161,24 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
 
         return
 
-      case 'session.info':
+      case 'session.info': {
+        const info = ev.payload
+
         patchUiState(state => ({
           ...state,
-          info: ev.payload,
-          // Flip from 'starting agent…' → 'ready' when the agent is live.
-          // Leave running/interrupted/error statuses alone.
+          info,
+          // agent just came online → flip the 'starting agent…' placeholder.
+          // leave running/interrupted/error statuses alone.
           status: state.status === 'starting agent…' ? 'ready' : state.status,
-          usage: ev.payload.usage ? { ...state.usage, ...ev.payload.usage } : state.usage
+          usage: info.usage ? { ...state.usage, ...info.usage } : state.usage
         }))
-        // Agent init is async in session.create, so the intro message may
-        // have been seeded with partial info (just model/cwd). Upgrade it
-        // in-place when the real session.info lands.
-        setHistoryItems(prev => prev.map(m => (m.kind === 'intro' ? { ...m, info: ev.payload } : m)))
+
+        // upgrade the seeded/partial intro row in-place with the real info
+        setHistoryItems(prev => prev.map(m => (m.kind === 'intro' ? { ...m, info } : m)))
 
         return
+      }
+
       case 'thinking.delta': {
         const text = ev.payload?.text