fix(web): stop 404 signout on post-signup sync and tolerate cache-only local edits

[posthog]

Summary

Fixes two offline-handling regressions introduced by the event-runtime cutover to calendar-owned contracts (#2017):

1. Post-signup sync 404 → surprise sign-out. syncLocalEventsToCloud fell back to a client-generated sentinel calendar id when it couldn't find a server-side local calendar, then POST /event against it. The backend can't resolve that id and throws CALENDAR_NOT_FOUND (404). handleErrorResponse treated a 404 on a data endpoint as an auth failure and signed the user out — right after they signed up. Now the sync only ever targets the real server local calendar; if it isn't available yet, the records stay on-device (untouched) for a later sync instead of being pushed with a bogus id. Separately, a 404 on a non-auth endpoint no longer forces sign-out — only genuine session failures (GONE/UNAUTHORIZED) do.

2. Event not found when editing offline. The optimistic layer resolves the edit target from the react-query cache, but LocalEventRepository reads IndexedDB. An id present in the cache but absent from the store (e.g. a materialized recurring-occurrence instance) threw Event not found. replace now upserts instead of throwing, so the edit persists rather than crashing.

Simplicity

Changes are minimal and local to the three affected modules: drop the sentinel fallback in the sync util, remove NOT_FOUND from the sign-out trigger, and make the local repository's replace tolerant of a missing record. No interface or contract changes.

Automated validation

Not run in this environment (no app/browser session). Covered by unit tests below.

Independent review

Self-reviewed the diff; the sentinel fallback, the sign-out trigger, and the throwing lookup were the three root causes and each is addressed at its source.

Test plan

• bun test src/api src/auth/google/util src/events/repositories src/common/utils/sync (packages/web) — 79 pass, 0 fail, including new cases:
  • sync keeps records on-device (no POST, no clear) when no server local calendar exists
  • handleErrorResponse does not sign out on a 404 data-endpoint response, and still does on UNAUTHORIZED
  • LocalEventRepository.replace upserts (instead of throwing) when the target is absent from the store, and preserves an input-provided calendar

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