Fix flaky embed tests: identity loading outlives jsdom teardown

[lsegal]

Fixes the flaky app/src/embed.test.ts failure where identity loading started by App outlived the jsdom environment and threw ReferenceError: localStorage is not defined after teardown (example run).

Root cause

App starts loadIdentity (IndexedDB → localStorage) in an effect. The embed tests waited only one setTimeout(0) tick, and one mounted root was never unmounted, so identity loading could still be running when Vitest tore down jsdom. The effect also didn't check whether it had been unmounted until after LocalProject.open, so a late identity result still opened a project, and a late failure became an unhandled rejection.

Changes

• app/src/App.tsx: the identity-loading effect ignores the result (or a rejection) of loadIdentity once the component has unmounted, and no longer opens a LocalProject after unmount.
• app/src/embed.test.ts:
  • wraps loadIdentity to track in-flight loads, and afterEach waits for them to settle before the test ends;
  • unmounts the second mounted root, which was previously leaked;
  • adds a regression test: identity loading that rejects after unmount does not open a project and does not produce an unhandled rejection. This test fails without the App.tsx change.

Tests

• vitest run src/embed.test.ts ran 20 times in a row: 20/20 passed with no unhandled errors.
• pnpm run lint and pnpm run test passed (13 files, 42 tests).

Changelog

None added: the project has no changelog.

UI

Not applicable. This doesn't change any UI.

Closes #5

Agents: claude-code (claude-opus-5-5)