Intermittent: embedded dashboard sometimes fails to render

# Intermittent: embedded dashboard sometimes fails to render (needs repro)

Slack thread: https://flowfuse.slack.com/archives/C0B8JS9KMLP/p1785961997022239?thread_ts=1785484819.381429&cid=C0B8JS9KMLP

## Symptom

Embedded (iframe) dashboards occasionally show the "sad face" error instead of rendering. Rare, transient, hit-and-miss. Not reliably reproduced. Opening the instance/dashboard directly once, then reloading the embed, has resolved it. An existing workaround is unreliable.

## Why the obvious cause doesn't hold

First guess: no session cookie → iframe redirects to `app.flowfuse.com` to authorize → that page is frame-busted → embed fails. But that's deterministic — it'd fail 100% of the time on a real first embed. The behavior is intermittent, so the framed redirect is at most part of it.

## Candidate causes

- **Third-party cookies (leading).** The dashboard session cookie is third-party in the embed (embed domain ≠ dashboard domain). Visibility depends on browser: Safari ITP, Chrome 3P-cookie phase-out, incognito, partitioning (CHIPS). Fits "works for most, fails unpredictably."
- **Framed authorize redirect.** Can't complete inside an iframe; explains why opening top-level once fixes it, but not the intermittency alone.
- **Timing/race.** Embed loads before the session is established.

## Needed to repro

Browser + version; normal vs incognito; third-party cookies allowed or blocked; is the embed cross-domain from the dashboard URL; had the user opened the instance before; steps + fail/success rate.

Scope is embedded/iframe only — directly-opened dashboards authorize fine via a normal top-level redirect. Previously flagged as the "Gate A" failure.