fix(web): keep earlier recurrence instances when opening an occurrence

[tyler-dane]

Summary

Opening a later weekday occurrence of a recurring series (e.g. Mon–Fri standup → click Thursday) made earlier instances disappear, even though the form still showed M–F selected.

Cause: useRecurrence rebuilds the RRULE on open and re-emits default INTERVAL=1. Google-style rules omit that default, so the mount effect treated serialization drift as a real edit, flipped the draft from preserve → series, suppressed sibling occurrences, and only showed forward draft previews from the clicked day.

Fix: Extend normalizeRecurrenceRule to treat default INTERVAL=1 as equivalent to an omitted interval (alongside the existing WKST strip), so open-for-edit stays preserve.

Simplicity

No separate simplify commit. The change is a one-line-class normalize extension plus a focused regression test. Sibling suppression and forward-only draft previews are left as-is — they are correct once a real recurrence edit exists.

Automated validation

Browser at http://localhost:9080 (anonymous IndexedDB):

1. Created Mon–Fri Daily standup at 9 AM for Aug 10–14, 2026 — all five instances visible
2. Clicked Thursday instance — Mon/Tue/Wed/Fri remained visible; form still showed M–F selected
3. No console errors

Independent review

Fresh diff-first review: CLEAN after follow-ups.

• Fixed: regression test now applies a real setDraft updater so suppressedSeriesIdForDraft asserts are meaningful
• Fixed: INTERVAL=1 stripped in either RRULE position (;INTERVAL=1 or :INTERVAL=1;)

Residual risk: other RRULE serialization drift beyond INTERVAL/WKST could still false-trigger a rewrite; not observed for the reported path.

Test plan

• bun test:web useRecurrence.test.ts — 12 pass
• bun run lint — pass (pre-existing unrelated warnings only)
• CI on HEAD fa36955: lint, type-check, knip, unit (core/web/backend/sync/scripts), e2e, CodeQL — all green
• Browser golden path above — PASS