Add seven plural forms no catalogue ever had

Found by widening the coverage check to plural() forms in every file rather
than the two being worked on. Seven counted strings in the Files view and the
event editor had never been in any of the nine catalogues, so they rendered in
English whatever language was chosen.

Not a regression from the recent work -- they have been missing since the
features landed, and every earlier scan looked at t("literal") sites and the
plurals of whichever file was in hand.

All nine languages, one commit rather than nine: this is a single gap in a
check rather than a translation pass, and splitting it per language would
suggest nine decisions where there is one.

web/src/locales/ja.ts

@@ -1299,8 +1299,20 @@ export const catalog: Catalog = {
     "{header} {op} \"{value}\"": "{header} が \"{value}\" を{op}",
     "{rule}, until {date}": "{rule}（{date} まで）",
     "{tests} → {actions}": "{tests} → {actions}",
+    // ── Third pass ──────────────────────────────────────────────────────
+    // Sentences that lib/ and store/ were building in English, and the two
+    // swipe labels that reach t() through a variable and so were invisible
+    // to a scan for t("literal"). See #259.
   },
   plurals: {
+    // ── Third pass ─────────────────────────────────────────────────────
+    "Delete {n} items": { other: "{n} 件を削除" },
+    "Delete {n} items?": { other: "{n} 件を削除しますか？" },
+    "Move {n} items": { other: "{n} 件を移動" },
+    "Move {n} items…": { other: "{n} 件を移動…" },
+    "The event runs {n} days longer than this shows.": { other: "この予定は表示よりも {n} 日長く続きます。" },
+    "{n} guests are not on this server, so there is no free/busy to read for them.": { other: "{n} 名の参加者はこのサーバーにいないため、空き情報を取得できません。" },
+    "{n} items selected": { other: "{n} 件を選択中" },
     // ── Third pass ─────────────────────────────────────────────────────
     "Every {n} days": { other: "{n} 日ごと" },
     "Every {n} months": { other: "{n} か月ごと" },