fix(desktop): close the backend update overlay on success; error on no-return

Three rough edges in the remote backend apply flow:
- On success the overlay dropped to IDLE, briefly re-rendering the pre-install
  'update available' view and then the generic 'you're all set' before settling.
  Close the overlay outright once the backend is confirmed back instead of
  bouncing through the idle view.
- If the backend never came back (a failed restart), the flow still reported
  success. waitForBackendReturn now returns whether the backend answered;
  finishBackendApply surfaces an error when it didn't.
- The up-to-date copy said 'you're running the latest version', conflating
  client and backend. Backend target now reads 'the backend is running the
  latest version' — the client's own version is a separate pill.

apps/desktop/src/i18n/en.ts

@@ -1237,6 +1237,7 @@ export const en: Translations = {
     unsupportedMessage: 'This version of Hermes can’t update itself from inside the app.',
     connectionRetry: 'Check your connection and try again.',
     latestBody: 'You’re running the latest version.',
+    latestBodyBackend: 'The backend is running the latest version.',
     allSetTitle: 'You’re all set',
     availableTitle: 'New update available',
     availableBody: 'A new version of Hermes is ready to install.',

apps/desktop/src/i18n/ja.ts

@@ -1378,6 +1378,7 @@ export const ja = defineLocale({
     unsupportedMessage: 'このバージョンの Hermes はアプリ内から自分を更新できません。',
     connectionRetry: '接続を確認してもう一度試してください。',
     latestBody: '最新バージョンを実行しています。',
+    latestBodyBackend: 'バックエンドは最新バージョンを実行しています。',
     allSetTitle: '準備完了',
     availableTitle: '新しい更新が利用可能',
     availableBody: '新しいバージョンの Hermes をインストールする準備ができています。',

apps/desktop/src/i18n/types.ts

@@ -937,6 +937,7 @@ export interface Translations {
     unsupportedMessage: string
     connectionRetry: string
     latestBody: string
+    latestBodyBackend: string
     allSetTitle: string
     availableTitle: string
     availableBody: string

apps/desktop/src/i18n/zh-hant.ts

@@ -1344,6 +1344,7 @@ export const zhHant = defineLocale({
     unsupportedMessage: '此版本的 Hermes 無法在應用程式內自行更新。',
     connectionRetry: '請檢查網路連線後重試。',
     latestBody: '您正在執行最新版本。',
+    latestBodyBackend: '後端正在執行最新版本。',
     allSetTitle: '已是最新版本',
     availableTitle: '有可用更新',
     availableBody: '新版 Hermes 已可安裝。',

apps/desktop/src/i18n/zh.ts

@@ -1424,6 +1424,7 @@ export const zh: Translations = {
     unsupportedMessage: '此版本的 Hermes 无法在应用内自行更新。',
     connectionRetry: '请检查网络连接后重试。',
     latestBody: '你正在运行最新版本。',
+    latestBodyBackend: '后端正在运行最新版本。',
     allSetTitle: '已是最新',
     availableTitle: '有可用更新',
     availableBody: '新版 Hermes 已可安装。',