Fix flaky embed.test.ts: identity loading outlives jsdom teardown

[lsegal]

app/src/embed.test.ts intermittently fails CI with an unhandled rejection after the jsdom environment is torn down:

ReferenceError: localStorage is not defined
 ❯ migrateLegacyDevice src/lib/identity.ts:204:18
 ❯ loadStoredDevice src/lib/identity.ts:227:26
 ❯ loadIdentity src/lib/identity.ts:452:27
 ❯ src/App.tsx:350:30
This error originated in "src/embed.test.ts" test file ... caught after test environment was torn down.

Example failing run: https://github.com/sssubscript/write/actions/runs/37154350731 (first attempt failed; the rerun passed with no code change). main has also had intermittent CI failures.

Cause

The embed tests mount App, which starts async identity loading (loadIdentity → IndexedDB → localStorage) in an effect. The tests only await a single setTimeout(0) tick, so that work can still be running when Vitest tears down jsdom, and the next localStorage access throws.

Acceptance criteria

• The embed tests no longer leave identity loading running past teardown, for example by waiting for it to settle before unmounting or by stubbing loadIdentity in these tests.
• Alternatively, or additionally, the identity loading effect in App ignores or cancels work after unmount.
• pnpm run test passes reliably with no unhandled rejections, repeated (e.g. 20 consecutive runs of embed.test.ts).

Addresses #2 and #4

[lsegal]

Starting work on this issue /glorp:45E62B60