fix(ui-tui): stabilize embedded dashboard chat gateway (#44528)

Cherry-picked from #39840 by @flyinhigh and rebased cleanly on main.

- Defer config fetch in createGatewayEventHandler until gateway.ready to
  avoid render-phase RPC that can mutate transcript state and trigger
  React error 301 in embedded dashboard PTYs.
- Use undici WebSocket fallback when globalThis.WebSocket is unavailable
  (Node attach mode and sidecar mirror sockets).
- Add regression tests for both fixes.

Co-authored-by: flyinhigh <flyinhigh@users.noreply.github.com>

ui-tui/src/app/createGatewayEventHandler.ts

@@ -219,11 +219,6 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
     agentsNudgedThisTurn = false
   }
 
-  // Kick off the config fetch eagerly at handler creation so the flag is
-  // resolved well before the first delegation of any real session (which
-  // only happens after gateway.ready + a user turn).
-  ensureAgentsNudgeConfig()
-
   const refreshDelegationStatus = (force = false) => {
     const now = Date.now()
 
@@ -312,6 +307,12 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
       applySkin(skin)
     }
 
+    // Kick off the config fetch once the gateway is actually ready. If handler
+    // construction does this during React render, a startup transport error can
+    // report through sys(), mutate transcript state, and trip React's
+    // "too many re-renders" guard in embedded dashboard PTYs.
+    ensureAgentsNudgeConfig()
+
     rpc<CommandsCatalogResponse>('commands.catalog', {})
       .then(r => {
         if (!r?.pairs) {