For the F4-C·F cue strengthening surfaces (tray tooltip + Inbox identity
counter), main + renderer need a single source of truth for "오늘 N번
잡아뒀다". Implements `NoteRepository.countToday(now?)` that computes the
half-open UTC interval covering the KST calendar date of `now` and counts
rows whose `created_at` falls inside.
`now` is injectable for deterministic tests across the KST/UTC boundary
(02:00 KST and 23:00 KST land on different UTC dates yet the same / a
different KST day). Four new cases cover empty DB, KST-day filtering,
KST-midnight crossover, and the default-arg branch.
cca3029b7e