Fix: localize the PWA install prompt

The PWA install prompt was hardcoded English regardless of the selected UI
language (and the large English block tripped Chrome's translate popup on
Android). Add a pwa_install namespace to all 17 locales, switch the component
to useTranslations, and move <PWAInstallPrompt /> from (main)/layout into
(main)/[locale]/layout so it renders inside the IntlProvider. The title keeps
the dynamic {appName}, so per-domain branding still applies.

locales/ja/common.json

@@ -1,4 +1,12 @@
 {
+  "pwa_install": {
+    "title": "{appName} をインストール",
+    "description": "素早いアクセスとオフライン対応のため、アプリをインストールしましょう。",
+    "not_now": "後で",
+    "install": "インストール",
+    "dont_remind": "今後表示しない",
+    "dismiss_aria": "インストールプロンプトを閉じる"
+  },
   "login": {
     "title": "ウェブメール",
     "username_label": "メールアドレス",