## Context
Phase 1 of removing the legacy `gridPosition` field from
`PageLayoutWidget` in favor of the new `position` discriminated union
(`grid` / `vertical-list` / `canvas`). This PR is purely additive —
`gridPosition` is still required and read everywhere; we just guarantee
that every widget now also has a non-null `position` so a follow-up PR
can drop `gridPosition` cleanly.
## Changes
- **Slow instance command**
`BackfillPageLayoutWidgetPositionSlowInstanceCommand` (2.1.0):
for every `core.pageLayoutWidget` row where `position IS NULL`, copies
`gridPosition`
into `position` with `layoutMode: 'GRID'`. Historically only grid
widgets used
`gridPosition`, so a single SQL update covers every existing row.
- **`PageLayoutDuplicationService`**: when duplicating a widget, also
forwards
`originalWidget.position` (was previously only forwarding
`gridPosition`).
- **Frontend default layouts** (10 `Default*PageLayout.ts` files): added
a `position`
sibling to every widget, matching the parent tab's `layoutMode` —
`VERTICAL_LIST` widgets
get `{ layoutMode, index }`, `CANVAS` widgets get `{ layoutMode }`
<img width="338" height="226" alt="Screenshot 2026-04-24 at 16 23 17"
src="https://github.com/user-attachments/assets/c3319318-f1b8-4271-96b4-196b209a1f5e"
/>
