Make sure the count for sending email blasts is accurate

[ArendPeter]

User: I was playing around with the feature of sending an email to only voters who haven't voted so far. The button said "Send 3 emails" even though 2 of the 3 voters had already voted. Ultimately, only 1 email was sent (which is correct), but the button name was confusing. Perhaps that button should just say "Send emails" for all cases.

This was reported in ticket 470

I think we should fix the button so the correct number is shown. I like having a count there to give myself confidence that I'm filtering it properly

[github-actions]

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[ArendPeter]

woops, didn't mean to assign myself. Someone else could totally work on this

[masiarek]

Opened #1532 — and the discrepancy is entirely in the code, no SendGrid access needed.

Both sides apply the same submitted filter; they apply it to different snapshots. The button counts electionRoll, an array fetched once when the Voters page mounts (useGetRolls in a useEffect with an empty dependency list — refetched only after editing a voter or clearing the list). The backend re-queries the roll at send time and filters again. Any ballot cast between page load and pressing the button makes the number wrong, which is your "Send 3 emails / 1 sent" exactly.

The PR refetches the roll when the dialog opens, and disables the button at zero — previously a stale positive count could be pressed against an empty target and the backend answered with a 400 ("All voters have voted"), an error for a state the UI shouldn't have offered.

Kept the count rather than dropping it, per @ArendPeter's reply on the confidence it gives.

One gap worth flagging: there is no test coverage of recipient selection at all — no backend test touches sendEmails, and no Playwright spec opens this dialog. Adding one needs a fixture with a partly-voted roll, which is more than this fix, but it is the kind of thing that would have caught this.