Remember the New Workspace isolation choice

The local/worktree choice was unstored component state and reset on every
remount of the New Workspace screen (e.g. after creating a workspace).
Persist it in the create-form preferences (FormPreferences.isolation).

Also rename the app-layer "backing" to "isolation" to match its UI label.
Isolation is the create-time intent; it stays distinct from workspaceKind,
the git-derived persisted property, and the wire's source.kind.

packages/app/src/i18n/resources/es.ts

@@ -859,7 +859,7 @@ export const es: TranslationResources = {
   newWorkspace: {
     title: "Nuevo espacio de trabajo",
     create: "Crear",
-    backing: {
+    isolation: {
       local: "Local",
       worktree: "Nuevo worktree",
       label: "Aislamiento",