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/zh-Hans.ts

@@ -1298,8 +1298,20 @@ export const catalog: Catalog = {
     "{header} {op} \"{value}\"": "{header} {op} \"{value}\"",
     "{rule}, until {date}": "{rule}，直到 {date}",
     "{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: "删除 {n} 个项目" },
+    "Delete {n} items?": { other: "要删除 {n} 个项目吗？" },
+    "Move {n} items": { other: "移动 {n} 个项目" },
+    "Move {n} items…": { other: "移动 {n} 个项目…" },
+    "The event runs {n} days longer than this shows.": { other: "此活动比这里显示的时间长 {n} 天。" },
+    "{n} guests are not on this server, so there is no free/busy to read for them.": { other: "有 {n} 位与会者不在此服务器上，因此无法读取他们的空闲/忙碌信息。" },
+    "{n} items selected": { other: "已选择 {n} 个项目" },
     // ── Third pass ─────────────────────────────────────────────────────
     "Every {n} days": { other: "每 {n} 天" },
     "Every {n} months": { other: "每 {n} 个月" },