feat(host-control-mode): localize the in-page dialog & badge

The host-control desync dialog and the "watching on your own" badge were
hardcoded English — content.js has no i18n loader. Background does, so it now
resolves the strings (GET_HCM_STRINGS: loadLocale + getMessage) and content
fetches them on init with English fallbacks, re-rendering the badge if it was
already shown. The dialog body is now generic (dropped the paused/jumped verb)
to keep translation clean across languages.

Adds 6 keys (HCM_DIALOG_TITLE/BODY/STAY/SOLO, HCM_BADGE_SOLO/RESYNC) translated
across all 15 popup locales.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/locales/ja.json

@@ -52,6 +52,12 @@
   "BADGE_GUEST": "ゲスト",
   "BADGE_DESYNCED": "ソロ",
   "TOOLTIP_PEER_DESYNCED": "単独で視聴中 — ホストのコマンドを無視しています",
+  "HCM_DIALOG_TITLE": "KoalaSync · ホストがこのルームを操作中",
+  "HCM_DIALOG_BODY": "このルームではホストのみが再生を操作できます。一緒に視聴を続けますか、それとも自分だけで視聴しますか？",
+  "HCM_DIALOG_STAY": "同期を維持",
+  "HCM_DIALOG_SOLO": "自分だけで視聴",
+  "HCM_BADGE_SOLO": "自分だけで視聴中",
+  "HCM_BADGE_RESYNC": "再同期",
   "NO_PEERS_CONNECTED": "接続しているメンバーはいません",
   "BTN_LEAVE_ROOM": "ルームを退室",
   "LABEL_SELECT_VIDEO": "ビデオを選択",