Add unit tests for useSessionStart hook (rate limit + identity parsing)

• 4h rate limit via chrome.storage.local.lastSessionStart
• userId read directly from storage (works pre-auth)
• createdDate → days_since_signup parse with Number.isFinite guard
• storage-write-before-send ordering (intentional, documents a ≤1 extra event/4h race window)

Author's own spec (tasks/phase1-spec.md AC2/AC4) calls out that runtime validation in staging is "still required" — a unit test of the hook itself catches half of that work without waiting on staging.

Suggested cases:

1. Skips when lastSessionStart is within 4h (now - lastSessionStart < SESSION_START_INTERVAL).
2. Fires when ≥ 4h has elapsed.
3. userId is undefined and the call still succeeds when storage has no user.
4. days_since_signup is null for an unparseable createdDate.
5. chrome.storage.local.set is called before sendLogEvent (race-window guarantee).

File: apps/extension/app/hooks/useSessionStart.ts:1-75