Fix recurring tasks syncing as one-time across providers

[deeqdev]

Summary

• preserve recurrence in Google metadata as machine RRULE text instead of humanized text
• accept both key:value and legacy key=value metadata formats and support legacy recurrence keys (repeat, rrule, recurrence_raw)
• improve recurrence parsing/normalization for legacy human strings (including interval/day/count/until patterns)
• preserve recurrence range details (DTSTART/TZID/WKST) during Microsoft serialize/deserialize
• allow Microsoft updates to re-apply recurrence payload (with due-date fallback start date) so one-time drift can be corrected

Root cause

Recurrence was written to Google notes in a human-readable form (for example: Every 2 weeks on Mon, Thu) that could not always be parsed back into RRULE, so Microsoft received no valid recurrence object and tasks were created/kept as one-time items.

Tests

• npm test
• npm run typecheck
• npm run lint

[deeqdev]

Superseded by #5, which is already merged into main with the latest recurrence fixes.