fix(web): reload the preview iframe through about:blank on retry

[pedrofrxncx]

Follows #5324 (self-heal preview iframe stuck on connection-refused), merged to main just before this run.

The bug

useIframeLoadRecovery's retry path (preview-iframe-recovery.ts) reassigns the iframe's src to the exact same URL string it already has to force a reload:

iframe.src = currentSrc;

But in the failure case this watchdog exists to recover from, the iframe never navigated away from that URL in the first place (it's stuck on the browser's connection-refused page while src still reads the sandbox URL). Reassigning an unchanged src string is a documented no-op in several browsers (Firefox in particular skips the navigation entirely when the value doesn't change; this is the well-known "iframe src reload hack is unreliable" gotcha). So the entire self-heal mechanism silently does nothing in those browsers — it schedules retries forever but the frame never actually reloads.

The fix

Bounce through about:blank before restoring the target URL, so the assignment always differs from the previous value and reliably triggers a real navigation:

iframe.src = "about:blank";
iframe.src = currentSrc;

Regression test

Added preview-iframe-recovery.test.ts, which fakes setTimeout/clearTimeout with a manually-flushed queue (the real watchdog/backoff delays are 10s/1s) and asserts the iframe's src assignment sequence on retry. Confirmed it fails against the pre-fix code (git stash + rerun) with the exact same-string no-op it's meant to catch, and passes with the fix.

To verify

bun test apps/web/src/components/sandbox/preview/preview-iframe-recovery.test.ts

Checks run locally

• bun run fmt
• bunx tsc --noEmit in apps/web
• targeted test above (green)

Full CI validates the rest.

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Summary by cubic

Fixes the preview iframe self-heal so retries trigger a real reload by bouncing through about:blank. Prevents endless retries on browsers that treat same-URL src assignments as no-ops (e.g., Firefox).

• Bug Fixes
  • Update useIframeLoadRecovery to set iframe.src = "about:blank" before restoring the target URL.
  • Add a regression test to assert the about:blank → target sequence with fake timers.

^{Written for commit a4e7ca1. Summary will update on new commits.}