e2e flake: Settings says "Your seat could not be read." after Checkout's return (billing-page) #775

Seen on #750's CI (run 41144092617, e2e shard 4/4, billing-page). The same code passed on its neighbours in the stack (#745, #753).

What failed: after Checkout's return goes home, the lane opens /settings and waits 30 s for "Always-on seat, … a month" and the "Add seats for others" button (crates/e2e/src/lanes/billing.rs:1672). The page showed "Your seat could not be read." for the whole wait (screenshot billing-page/5-billing.png in the e2e-scratch-4 artifact). Two checks failed: "their Billing shows their seat…" and "Billing's Add seats for others: it is on the page".

What the logs say: every GET /api/seat in the kept node logs answered 200 OK, across all 14 boots, so the seat read didn't fail at the cell as far as the logs show. cell/shell/settings.js reads /api/seat once in a Promise.all with .catch(() => null). A null seat renders "Your seat could not be read." (billing.js:320), and nothing reads it again until Settings reopens. So whatever made that one read fail (a request that never reached a node, a node boot, a session refresh), the page keeps the error and the wait can't recover.

Not yet known: why the browser's read failed while the node logged only 200s.

Not touched by the session-archive stack. Filed so the class can be root-caused rather than rerun.