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/ko/sidebar.json

@@ -45,6 +45,7 @@
     "removeFromFavorites": "즐겨찾기에서 제거",
     "editSessionName": "세션 이름 직접 편집",
     "deleteSession": "이 세션 영구 삭제",
+    "activeSessionIndicator": "최근 활성 세션 (지난 10분)",
     "save": "저장",
     "cancel": "취소",
     "openCommandPalette": "명령 팔레트 열기"