stop the app editor crashing on a grid-less draft (#10203)

* fix(apps): default a missing grid so the app preview does not crash

* fix(apps): migrate the adopted draft so the editor matches the viewer

* docs: tighten grid-normalization comments to durable constraints

* test: drop the migrateApp grid-default test

frontend/src/lib/components/apps/editor/AppEditor.svelte

@@ -127,7 +127,15 @@
 	if (appDraftHandle) UserDraft.stopSync('app', appDraftPath)
 	// Prefer a prior-session autosave over the prop (the route seeds an empty
 	// template on `new_draft=true`; it calls `UserDraft.remove` to force a reset).
-	const stateApp = $state(untrack(() => appDraftHandle?.draft ?? app))
+	// A draft is a stored value that can predate any schema migration, so whichever
+	// value is adopted needs the same normalization as the prop above.
+	const stateApp = $state(
+		untrack(() => {
+			const adopted = appDraftHandle?.draft ?? app
+			migrateApp(adopted)
+			return adopted
+		})
+	)
 	const appStore = writable<App>(stateApp)
 	// Mirror `stateApp` mutations into the autosave cell. The first mirror is the
 	// seed — `acquireEntry`'s `skipNextWrite` swallows it; later writes POST.

frontend/src/lib/components/apps/migrateApp.ts

@@ -4,10 +4,11 @@ import { allItems } from './editor/appUtilsCore'
 
 /**
  * Normalize an `App` in place to the current schema: default `hiddenInlineScripts`
- * type, migrate the legacy `doNotRecomputeOnInputChanged` flag, and default
- * `fullHeight` on every grid item. Lives in its own light module (no app-editor
- * component imports) so non-editor callers — e.g. the localStorage→DB draft
- * migration — can reuse it without pulling the whole `apps/utils` graph.
+ * type, migrate the legacy `doNotRecomputeOnInputChanged` flag, default a missing
+ * `grid`, and default `fullHeight` on every grid item. Lives in its own light
+ * module (no app-editor component imports) so non-editor callers, e.g. the
+ * localStorage→DB draft migration, can reuse it without pulling the whole
+ * `apps/utils` graph.
  */
 export function migrateApp(app: App) {
 	;(app?.hiddenInlineScripts ?? []).forEach((x) => {
@@ -22,6 +23,14 @@ export function migrateApp(app: App) {
 		}
 	})
 
+	// A stored app value can have no `grid` at all: a persisted draft row is the
+	// confirmed case, and the editor renders a draft in place of the deployed
+	// value. The grid components dereference it unguarded, so normalize it here,
+	// the one hook every load path runs through.
+	if (!Array.isArray(app.grid)) {
+		app.grid = []
+	}
+
 	allItems(app.grid, app.subgrids).forEach((x) => {
 		gridColumns.forEach((column: number) => {
 			if (x?.[column]?.fullHeight === undefined) {