The cut line on Next up moves while a row is dragged, before the save #8858

Follows #8704. The cut line is drawn on the server; a drag saves and reloads the page (location.reload() in src/web/pages/next.ts), so the line only moves after the round trip.

Done looks like:

• [x] Each row carries its estimate's upper bound in a data attribute and the list carries the week's remaining capacity; while a row is dragged over the list the line re-places itself from those, and rows below it read as next week.
• [x] On drop the save happens as today and the server's line matches what was shown (a test feeds the same numbers to weekView and the client rule).
• [x] No layout shift at rest; with no script the page is as today.

Files: src/web/pages/next.ts, src/flow/week.ts, a test

Reviewed pull #8894 at its head: ticked Each row carries its estimate's upper bound in a data attribute and the list carries the week's remaining capacity; while a row is dragged over the list the line re-places itself from those, and rows below it read as next week.; On drop the save happens as today and the server's line matches what was shown (a test feeds the same numbers to weekView and the client rule).; No layout shift at rest; with no script the page is as today..