Dashboard: setDashboard mutations clobber each other when fired back-to-back #2216

Symptom

Two setDashboard(...) calls in quick succession (eg. toggle bordered on group A, then add a tab to group B) can leave the backend with only the second mutation's effect. The first change appears to "save" in the UI but is lost on reload.

Surfaced while writing the DashboardContainer e2e round-trip in PR #2205. The save-and-reload test has been narrowed to a single mutation per step to work around this; the bordered-toggle round-trip is now covered only by the in-page test (PR #2205, line 78) and not by the navigate-away-and-back round-trip.

Root cause (confirmed via code reading)

setDashboard for a remote dashboard calls updateDashboard.mutate(newDashboard) (no optimistic cache update). The component still reads dashboard from the React Query cache until invalidateQueries -> refetch completes.

Two consecutive setDashboard(produce(dashboard, ...)) calls both produce from the same pre-mutation snapshot. Both PATCH /api/dashboards/{id}. The second PATCH overwrites the first because the body it sends derives from a snapshot that doesn't yet include the first mutation's change.

packages/app/src/dashboard.ts:151-181 (setDashboard).
packages/app/src/dashboard.ts:213+ (useUpdateDashboard).

Fix options

1. Optimistic update in useUpdateDashboard.onMutate: write the new dashboard into the cache immediately so the next produce derives from up-to-date state. Roll back in onError.
2. Serialize mutations through a queue (mutation chain) so the second waits on the first.
3. Switch the local component state to be the source of truth, with a debounced flush to the backend.

Option 1 is the smallest change and matches typical React Query patterns.

Acceptance

• E2E test: toggle bordered on group A, add a tab to group B in succession, navigate away, navigate back. Both changes survive.
• Existing in-page tests (PR test(e2e): cover unified DashboardContainer (collapse, border, tabs, drag) #2205 lines 48, 78) keep passing.
• The narrowed round-trip in PR test(e2e): cover unified DashboardContainer (collapse, border, tabs, drag) #2205 (line 231) can be re-extended to include a bordered toggle on group A.