Bug Description

When using custom retention brackets, cells whose time window has not yet fully elapsed
show 0% instead of being marked as in-progress (grayed out). This is misleading because
it implies zero retention when data collection is simply incomplete.

For example, with brackets [1, 3, 4, 8] weeks and a cohort starting Feb 1, 2026, the
"weeks 9–16" bracket window doesn't close until May 24, 2026. Yet the cell shows 0%
today (May 16) rather than indicating the period is still open.

This behavior is inconsistent with individual period columns, which correctly gray out
trailing cells when data collection is still in progress.

How to reproduce

1. Create a retention insight using weekly period with custom brackets (e.g. [1, 3, 4, 8]).
2. Set the date range to the last 4 months.
3. Observe the column corresponding to the longest bracket (e.g. "weeks 9–16") for
   cohort rows where cohort_start + bracket_end_week × 7 > today.
4. Those cells show 0% instead of being flagged as in-progress.

Additional context

Individual period columns at the trailing edge of the table are correctly marked as
in-progress when data is incomplete. The same logic does not appear to apply to custom
bracket columns — a cell shows as complete (0%) as soon as any data could theoretically
have been recorded, even if the bracket's full window hasn't elapsed.

Suggested fix: a bracket cell should be treated as in-progress when
cohort_start + bracket_end_week × 7 > today.

Debug info

- [x] PostHog Cloud, Debug information: Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/019e3325-fa7b-7d95-b8d7-3e7b67169570?t=4066 Admin: http://go/adminOrgUS/019b4f1e-a2c0-0000-aecd-78a9ab9544bc (project ID 271787)
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: n.a.
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: n.a.