Confirm before editing a poll option with existing votes

[leahpeker]

Context

Follow-up to #1393 (fixes #1300): the backend now clears all PollVotes on a poll option whenever its datetime is edited, to stop members being silently auto-RSVP'd to a date they never voted for. That fix is backend-only — the frontend has no UI yet for editing a poll option's datetime, and once it's added, editing an option that already has votes will silently wipe them with no warning.

What needs to happen

When a host edits a poll option that already has one or more votes, show a confirmation modal before submitting the change, warning that the option's existing votes will be deleted. Only options with yes_count > 0 (or equivalent) need the confirmation — editing an unvoted option can proceed without it.

Acceptance criteria

• Editing a poll option with existing votes shows a confirmation modal warning that votes on that option will be deleted
• Confirming the modal proceeds with the edit; canceling leaves the option and its votes unchanged
• Editing a poll option with zero votes does not show the modal