fix(captures): hide macOS-only copy when running on Windows / Linux

Two surfaces leaked macOS-specific copy onto other platforms:

1. The Input Monitoring + Accessibility rows in the readiness
   checklist rendered everywhere. On Windows/Linux the Rust permission
   stubs return true, so the rows showed as permanent green checkmarks
   with copy like "macOS allows Voicebox to detect your global
   shortcut." — nonsense when you're on Windows. Gate both rows on a
   userAgent-based isMacOS check so they only render where the
   underlying TCC permission actually exists.

2. The global-shortcut setting description ended with "macOS will ask
   for Input Monitoring permission the first time you turn this on."
   That sentence rendered on every platform. The readiness checklist
   already surfaces the TCC requirement at the right moment on macOS,
   so the description doesn't need the platform note — drop it from
   en / ja / zh-CN / zh-TW.

Other macOS strings (AccessibilityNotice, InputMonitoringNotice, their
"stillMissing" hints) are already gated behind the Rust permission
booleans returning false, which never happens on Windows/Linux, so they
stay inert without further changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

app/src/i18n/locales/ja/translation.json

@@ -851,7 +851,7 @@
         "description": "グローバルショートカットでマシン上のどこからでもキャプチャできます。",
         "globalShortcut": {
           "title": "グローバルショートカット",
-          "description": "ショートカットを押し続けるとマシン上のどこからでも録音できます。離すと文字起こしが行われます。最初に有効化したとき、macOS から入力監視の許可を求められます。"
+          "description": "ショートカットを押し続けるとマシン上のどこからでも録音できます。離すと文字起こしが行われます。"
         },
         "pushToTalk": {
           "title": "プッシュトゥトーク用ショートカット",