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/auditColumns.tsx

@@ -1,8 +1,7 @@
 import type { TFunction } from "i18next";
-import { relativeTime, formatDate } from "../lib/format";
-import i18n from "../i18n";
 import type { AuditRow } from "../lib/api";
 import type { Column } from "./ui/DataTable";
+import { RelativeTime } from "./ui/RelativeTime";
 
 function actionLabel(t: TFunction, action: string): string {
   // Canonical key → localized label; falls back to the raw key for any unmapped action.
@@ -44,13 +43,7 @@ export function auditColumns(
       sortValue: (r) => r.created_at ?? "",
       cardPrimary: true,
       render: (r) => (
-        <span
-          className="text-muted tabular-nums"
-          title={r.created_at ? formatDate(r.created_at, i18n.language) : ""}
-        >
-          {/* `|| "—"` not a null check: relativeTime is empty for a missing OR unreadable date. */}
-          {relativeTime(r.created_at) || "—"}
-        </span>
+        <RelativeTime at={r.created_at} seconds empty="—" className="text-muted tabular-nums" />
       ),
     },
     {