feat(testing): smoke-test script + VITE_FORCE_BOOT_STALL gate

- App.tsx and main.tsx ErrorBoundary gate rendererReady() on
  VITE_FORCE_BOOT_STALL build env var. When set, the ping is suppressed
  so the main-process boot timer fires (exercising the renderer-stalled
  recovery path without hand-editing source).
- vite-env.d.ts declares the env var type so TS doesn't complain.
- scripts/smoke-recovery.sh automates scenarios 2 (bad URL → load-failed)
  and 4 (forced stall → renderer-stalled), grepping stderr for the
  [recovery] entered: ... lines added by the prior commit. Backs up and
  restores the user's instance-url.json. Manual scenarios documented in
  the script header.

packages/web/src/main.tsx

@@ -23,6 +23,9 @@ class ErrorBoundary extends React.Component<
     // Renderer is alive enough to show the fallback UI — disarm the boot timer.
     // Without this, the timer fires 20s after a caught render error and
     // overrides the in-app error UI with native recovery, which is wrong.
+    // Gated on VITE_FORCE_BOOT_STALL so the smoke harness can suppress both
+    // ping paths simultaneously when testing the renderer-stalled recovery path.
+    if (import.meta.env.VITE_FORCE_BOOT_STALL) return;
     if (typeof window.backspace?.rendererReady === 'function') {
       window.backspace.rendererReady();
     }