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/en.ts

@@ -839,7 +839,7 @@ export const en = {
   newWorkspace: {
     title: "New workspace",
     create: "Create",
-    backing: {
+    isolation: {
       local: "Local",
       worktree: "New worktree",
       label: "Isolation",