fix(i18n): every counted string is a plural pair, so one reads singular

"1 seasons", "Synced 1 subscriptions", "1 video(s) were skipped": 21 English
strings carried {{count}} in one plain key and read plural at one. Each is a
_one/_other pair now (Hungarian keeps one form for both), and so is every other
counted string, so the next wording change is a plural too. A contract test
holds the rule for every locale file.

frontend/src/i18n/plurals.test.ts

@@ -6,6 +6,20 @@ import { describe, expect, it } from "vitest";
 
 import i18n from "./index";
 
+const mods = import.meta.glob("./locales/*/*.json", { eager: true }) as Record<
+  string,
+  { default: Record<string, unknown> }
+>;
+
+// Every leaf string as [dotted key, value].
+function leaves(obj: Record<string, unknown>, prefix = ""): [string, string][] {
+  return Object.entries(obj).flatMap(([k, v]) => {
+    const key = prefix ? `${prefix}.${k}` : k;
+    if (v && typeof v === "object") return leaves(v as Record<string, unknown>, key);
+    return typeof v === "string" ? [[key, v] as [string, string]] : [];
+  });
+}
+
 describe("counted copy", () => {
   it("the share link's expiry is singular for one day", async () => {
     await i18n.changeLanguage("en");
@@ -22,3 +36,26 @@ describe("counted copy", () => {
     expect(i18n.t("downloads.share.views", { count: 0 })).toBe("0 views");
   });
 });
+
+// E31: "1 seasons", "Synced 1 subscriptions" — a counted string written as one plain key reads plural at
+// one. So the rule is held for every string, not key by key: a value that carries {{count}} is a plural
+// pair (`_one` / `_other`), even where the two read the same today, so the next wording is a plural too.
+describe("every counted string is a plural pair", () => {
+  it("no {{count}} in a key without a plural suffix, in any language", () => {
+    const bare = Object.entries(mods).flatMap(([path, mod]) =>
+      leaves(mod.default)
+        .filter(([k, v]) => v.includes("{{count}}") && !/_(zero|one|two|few|many|other)$/.test(k))
+        .map(([k]) => `${path.replace(/^.*locales\//, "")}: ${k}`),
+    );
+    expect(bare).toEqual([]);
+  });
+
+  it("reads singular at one", async () => {
+    await i18n.changeLanguage("en");
+    expect(i18n.t("plex.seasons", { count: 1 })).toBe("1 season");
+    expect(i18n.t("plex.seasons", { count: 3 })).toBe("3 seasons");
+    expect(i18n.t("channels.notify.synced", { count: 1 })).toBe("Synced 1 subscription");
+    expect(i18n.t("playlists.pushPartial", { count: 1 })).toBe("Synced, but 1 video was skipped");
+    expect(i18n.t("playlists.pushPartial", { count: 2 })).toBe("Synced, but 2 videos were skipped");
+  });
+});