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/ar.ts

@@ -833,7 +833,7 @@ export const ar: TranslationResources = {
   newWorkspace: {
     title: "مساحة عمل جديدة",
     create: "يخلق",
-    backing: {
+    isolation: {
       local: "محلي",
       worktree: "شجرة عمل جديدة",
       label: "العزل",

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",

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",

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

@@ -858,7 +858,7 @@ export const fr: TranslationResources = {
   newWorkspace: {
     title: "Nouvel espace de travail",
     create: "Créer",
-    backing: {
+    isolation: {
       local: "Local",
       worktree: "Nouveau worktree",
       label: "Isolation",

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

@@ -851,7 +851,7 @@ export const ru: TranslationResources = {
   newWorkspace: {
     title: "Новое рабочее пространство",
     create: "Создавать",
-    backing: {
+    isolation: {
       local: "Локально",
       worktree: "Новый worktree",
       label: "Изоляция",

packages/app/src/i18n/resources/zh-CN.ts

@@ -824,7 +824,7 @@ export const zhCN: TranslationResources = {
   newWorkspace: {
     title: "新建 workspace",
     create: "创建",
-    backing: {
+    isolation: {
       local: "本地",
       worktree: "新建 worktree",
       label: "隔离",