[scheduler] Timezone Issue 5: Make recurring event expansion and comparisons timezone-consistent

[rita-codes]

• Analyze the recurring events engine to identify where rules are interpreted, occurrences are expanded, and date comparisons are made.
• Define and implement a clear rule: interpret the recurrence rule and generate occurrences in the event’s original timezone, then convert each occurrence’s start/end to the render timezone for UI.
• Ensure all recurrence-related comparisons (by day/range) are performed consistently with this rule.
• Reuse the same “convert back to original timezone” logic when updating single occurrences via drag & drop or popover.
• Add tests for recurring events crossing DST, with different original vs render timezones, and for updating individual occurrences.