When a record has no associated `pageLayoutId`, the FE falls back to a
hardcoded default page layout (`DEFAULT_*_RECORD_PAGE_LAYOUT`). These
mocks use non-UUID ids for the layout, tabs, and widgets
(e.g. `default-person-page-layout`).
Today nothing distinguishes these fallbacks from real layouts at edit
time. Clicking "Edit layout" flips the global customization mode,
`PageLayoutRecordPageCustomizationSessionRegistrationEffect` registers
the mock id, and on Save `useSaveLayoutCustomization` fires
`UpdatePageLayoutWithTabsAndWidgets` with those mock ids — the BE
rejects them and we end up in a partial-save state (nav / command menu
commit while the page layout fails).
This PR keeps fallback record page layouts in read mode even when
global layout-edition mode is on, and defends the save loop so mock
ids can never be sent to the BE.
## Non editable "base" record page layout (=mock)
<img width="1508" height="616" alt="Screenshot 2026-04-24 at 10 35 02"
src="https://github.com/user-attachments/assets/34b093d1-f4bb-4076-ab8c-ce97ecb945a4"
/>
## Editable record page layout
<img width="1512" height="597" alt="Screenshot 2026-04-24 at 10 35 20"
src="https://github.com/user-attachments/assets/5ff6f3ff-79a5-4e6f-aa1c-01c7e09273b3"
/>
