i18n(sidebar): translate the folder-scan strings

The sidebar's "Scan this folder" entry and its vanished-folder warning were the
last 2 [TODO: Translate] keys, so grep -c "TODO: Translate" locales/*.json is 0
everywhere again.

  scanFolder              zh-CN 扫描此文件夹 · zh-TW 掃描此資料夾 · ja このフォルダをスキャン
                          ko 이 폴더 스캔 · fr Analyser ce dossier · de Diesen Ordner scannen
                          es Escanear esta carpeta · ru Сканировать эту папку · he סרוק תיקייה זו
  scanFolderResult.missing
                          reused verbatim from each locale's existing
                          renameFolderResult.missing / deleteFolderResult.missing — the situation
                          is identical ("the folder vanished before the action ran"), so a third
                          variant of the same sentence would only be a translation drift risk.

The label follows each locale's "Check for updates in this folder" phrasing
(检查此文件夹的更新 → 扫描此文件夹, Vérifier les mises à jour dans ce dossier → Analyser ce
dossier) so the two refresh entries in the same menu read as a pair.
docs/i18n-translation-guidelines.md gains the two rows in "Scoped scan and root
availability" plus the updated status note.

Verified: python scripts/sync_translation_keys.py --dry-run reports no pending
changes, pytest tests/i18n 20 passed, frontend 1495 passed.

locales/ja.json

@@ -1483,9 +1483,9 @@
             "none": "このフォルダのすべての{type}sは最新です",
             "error": "フォルダの{type}アップデート確認に失敗しました: {message}"
         },
-        "scanFolder": "[TODO: Translate] Scan this folder",
+        "scanFolder": "このフォルダをスキャン",
         "scanFolderResult": {
-            "missing": "[TODO: Translate] This folder no longer exists on disk. Refresh the sidebar and try again."
+            "missing": "このフォルダはディスク上に存在しません。サイドバーを再読み込みしてからもう一度お試しください。"
         }
     },
     "statistics": {