fix(tui): first-run setup preflight + actionable no-provider panel

- tui_gateway: new `setup.status` RPC that reuses CLI's
  `_has_any_provider_configured()`, so the TUI can ask the same question
  the CLI bootstrap asks before launching a session
- useSessionLifecycle: preflight `setup.status` before both `newSession`
  and `resumeById`, and render a clear "Setup Required" panel when no
  provider is configured instead of booting a session that immediately
  fails with `agent init failed`
- createGatewayEventHandler: drop duplicate startup resume logic in
  favor of the preflighted `resumeById`, and special-case the
  no-provider agent-init error as a last-mile fallback to the same
  setup panel
- add regression tests for both paths

ui-tui/src/app/interfaces.ts

@@ -197,6 +197,7 @@ export interface GatewayEventHandlerContext {
     colsRef: MutableRefObject<number>
     newSession: (msg?: string) => void
     resetSession: () => void
+    resumeById: (id: string) => void
     setCatalog: StateSetter<null | SlashCatalog>
   }
   system: {
@@ -206,6 +207,7 @@ export interface GatewayEventHandlerContext {
   }
   transcript: {
     appendMessage: (msg: Msg) => void
+    panel: (title: string, sections: PanelSection[]) => void
     setHistoryItems: StateSetter<Msg[]>
   }
 }