feat: add --until, today/yesterday keywords; harmonize date flags

[DTTerastar]

Summary

Part 1 of 3 in a cross-repo date-flag harmonization across the quantcli export CLIs (crono / liftoff / withings). Liftoff already had `--since` with relative + absolute values; this PR extends it to:

• Accept `today` and `yesterday` as values (previously only the `workouts show` argument supported these keywords).
• Add a matching `--until VALUE` flag that is inclusive of the named day.
• Use a single shared `parseDateValue` helper for `--since`, `--until`, and `workouts show ` so all three accept the same vocabulary.

```
--since VALUE inclusive lower bound
--until VALUE inclusive upper bound (new)
VALUE: today | yesterday | YYYY-MM-DD | Nd/Nw/Nm/Ny
```

One semantic change worth flagging

`--since 30d` previously meant "30 days ago at this exact moment" (`time.Now().AddDate(0,0,-30)`). It now means "midnight of the day 30 calendar days ago". Same as how `--since 2025-01-01` already worked. Matches user intent better — running the same command at 11:59pm vs 12:01am should not return different windows.

Test plan

• `go build ./...` and `go vet ./...` pass
• `workouts list --since yesterday --until today` returns yesterday's workout
• `workouts show today` correctly reports no workout when today is empty
• `workouts list --since 3d --until 2026-04-21` works (empty window when ranges don't overlap, as expected)
• Reviewer: confirm `bodyweights list --since 6m --until 1m` returns a sensible historical window

Follow-ups

• crono-export-cli: needs the bigger refactor — `--today`/`--days`/`--start`/`--end` removed in favor of `--since`/`--until`.
• withings-export-cli: same shape as this PR.

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