Dashboard: success snackbar on layout save - #78072

* add staging layer to WidgetDashboard

WidgetDashboardProvider now keeps an internal staging copy of the
layout. In-progress mutations (drag, resize, insert, setAttributes)
mutate staging only; the consumer's onLayoutChange fires once on
commit. Adds hasUncommittedChanges (deep-equal between committed and
staging via fast-deep-equal) plus commitLayout and cancelLayout
actions. Auto-flips edit mode on when the layout becomes empty.

Existing tests are updated to follow the staged commit flow (insert
or setAttributes followed by Done) and the actions harness now uses
a non-empty layout so the auto-edit effect does not interfere.

* wire Done and Cancel to commit and cancel actions

Replaces the placeholder console.log handlers in the dashboard
toolbar with the staging-layer commit and cancel actions. Done
publishes the staged layout to the consumer; Cancel discards it
and exits edit mode without firing onLayoutChange.

* fix icon property type: IconType

* switch useDashboardLayout to a named export

Aligns the hook with the rest of the route's named-export
convention (the local barrel and stage.tsx already imported it as
named via re-export). Drops the now-stale test under the shared
hooks/test directory; the test moves next to the source in a
follow-up.

* colocate useDashboardLayout test with its source

Moves the unit test alongside the hook implementation under
hooks/use-dashboard-layout/test/, so each hook owns its tests
and the shared hooks/test/ directory disappears.

* ignore placement.order in uncommitted-changes diff

The grid model assigns explicit `order: 0, 1, …` to each item after
the first drag. A swap followed by an undo restores the visible
arrangement but leaves explicit orders in staging, while the
committed layout never had any. Deep-equal flagged this as dirty
and the toolbar treated it as a pending change.

Canonicalize both layouts before comparison: sort by
`placement.order` (falling back to the array index when omitted),
then strip `order` since the array position now encodes it. Adds
a regression test that simulates a swap and its undo.

* disable Done when there are no uncommitted changes

The Done button now reads `hasUncommittedChanges` from the dashboard
context and renders disabled when staging matches committed. Avoids
the no-op confirmation path when the user enters Customize and exits
without touching the layout.

Replaces the toggle-on-click test with a Customize-only assertion
plus an explicit "Done is disabled when there are no staging
changes" check.

* clarify onLayoutChange documentation

The block doc for WidgetDashboardProps and the field doc for
onLayoutChange were stale: the former mentioned a "reset replaces
the committed layout from outside" path that does not flow through
this callback, and the latter still described "every layout
mutation". With the staging layer in place, onLayoutChange only
fires from commitLayout (the Done action). Documents the actual
contract.

* canonicalize layout before publishing on commit

commitLayout was publishing the raw stagingLayout, while
hasUncommittedChanges already compared canonicalized copies. The
persisted payload accumulated redundant placement.order values that
the comparison treated as implicit; the publish form and the
compare form had drifted.

Pipe the staged layout through canonicalize() before invoking the
consumer's onLayoutChange so the persisted shape is sorted by order
with `order` stripped, matching the canonical comparison form.
Updates the canonicalize doc to describe the dual use and adds a
test that pins the publish payload.

* Dashboard: success snackbar on layout save (#78072)

* show success snackbar when the dashboard layout is saved

Wraps the dashboard route's onLayoutChange handler so it dispatches
a success notice on @wordpress/notices after persisting. The user
gets a "Layout saved." snackbar on Done, matching the feedback
pattern used by other admin surfaces (e.g., experimental features
toggles).

* Reword saving dashboard message

* udpate npm lock file