feat(sidebar): tooltip for the active-session indicator dot (#782)

The pulsing green dot next to a session row signals that the session
had activity in the last 10 minutes, but the meaning was undocumented.
Hovering it now shows a translated tooltip, and an aria-label exposes
the same text to screen readers.

Uses the existing shared Tooltip component (portal-positioned, so it
is not clipped by the sidebar overflow). Translation key added to all
eight sidebar locale files (en, de, it, ja, ko, ru, tr, zh-CN).

src/i18n/locales/ja/sidebar.json

@@ -45,6 +45,7 @@
     "removeFromFavorites": "お気に入りから削除",
     "editSessionName": "セッション名を手動で編集",
     "deleteSession": "このセッションを完全に削除",
+    "activeSessionIndicator": "最近アクティブなセッション（過去10分以内）",
     "save": "保存",
     "cancel": "キャンセル",
     "openCommandPalette": "コマンドパレットを開く"