feat(design): the Copy rule — names, verbs, dates and units (E28 S3g)

The owner's picks (all the recommended options) become DESIGN.md's Copy
chapter and a sweep of both languages (742 values, no key drift):

- One name per feature: the rail's label, which may shorten it (Alerts
  -> Notifications, Fiókok -> Felhasználók, Download Center -> Downloads,
  Channel manager -> Channels); one "sync from YouTube" verb.
- Four verbs, four meanings (Delete / Remove / Dismiss / Discard, and
  their Hungarian pairs), checked against what each action does.
- Sentence case, the app's places capitalised; "…" on a command that
  asks first (a popover or form title got its own key without it);
  one-sentence messages without a stop or a tick, "Couldn't …"; curly
  quotes.
- Dates only from lib/format.ts, in the UI language: formatDateTime
  names the month and stops at the minute (it printed "9/27/2026,
  8:30:14 AM"), formatDateTimeSec for the log, the audit and the
  recorder; ui/RelativeTime pairs "3d ago" with the exact minute on
  hover. local/date-format refuses toLocale*String and Intl date
  formatters elsewhere.
- Units from Intl, short (formatUnit): runtimes, totals, the Scheduler
  countdown, the stats range, the player's tap skip. "2h 13m" was
  English in both languages.
- src/i18n/copyRule.test.ts holds the machine-readable part (quotes,
  "...", ticks, bare unit letters).

frontend/src/components/ui/RelativeTime.dom.test.tsx

@@ -0,0 +1,40 @@
+// @vitest-environment jsdom
+//
+// docs/DESIGN.md "Copy": a relative time in a list carries the exact moment on hover.
+import { afterEach, describe, expect, it } from "vitest";
+import { cleanup, render } from "@testing-library/react";
+import { formatDateTime, formatDateTimeSec, relativeTime } from "../../lib/format";
+import { RelativeTime } from "./RelativeTime";
+
+afterEach(cleanup);
+
+const THREE_DAYS_AGO = new Date(Date.now() - 3 * 86400_000).toISOString();
+
+describe("RelativeTime", () => {
+  it("says how long ago, and names the exact minute on hover", () => {
+    const { container } = render(<RelativeTime at={THREE_DAYS_AGO} />);
+    const el = container.querySelector("time")!;
+    expect(el.textContent).toBe(relativeTime(THREE_DAYS_AGO));
+    expect(el.getAttribute("title")).toBe(formatDateTime(THREE_DAYS_AGO));
+    expect(el.getAttribute("dateTime")).toBe(THREE_DAYS_AGO);
+  });
+
+  it("takes epoch millis, and the log form with seconds", () => {
+    const ms = Date.parse(THREE_DAYS_AGO);
+    const { container } = render(<RelativeTime at={ms} seconds />);
+    expect(container.querySelector("time")!.getAttribute("title")).toBe(
+      formatDateTimeSec(THREE_DAYS_AGO),
+    );
+  });
+
+  it("shows the placeholder for a missing or unreadable moment, never a bare title", () => {
+    const { container } = render(
+      <>
+        <RelativeTime at={null} empty="—" />
+        <RelativeTime at="not a date" />
+      </>,
+    );
+    expect(container.textContent).toBe("—");
+    expect(container.querySelector("time")).toBeNull();
+  });
+});