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/Stats.tsx

@@ -4,7 +4,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { History, Pause, Play, RefreshCw } from "lucide-react";
 import { api, type AdminQuotaRow, type Me } from "../lib/api";
-import { formatEta, formatNumber, quotaActionLabel } from "../lib/format";
+import { formatEta, formatNumber, formatUnit, quotaActionLabel } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { useWizard } from "./wizardContext";
@@ -259,7 +259,7 @@ function SystemStats() {
           className="flex items-center gap-1.5"
           options={RANGES.map((d) => ({
             value: String(d),
-            label: t("stats.rangeDays", { count: d }),
+            label: t("stats.rangeDays", { d: formatUnit(d, "day") }),
           }))}
           value={String(days)}
           onChange={(v) => setDays(Number(v))}
@@ -286,7 +286,7 @@ function SystemStats() {
           <div className={`${SURFACE.card} p-3 mb-4`}>
             <div className="text-xs text-muted mb-2">
               {t("stats.dailyTotal", {
-                count: data.range_days,
+                d: formatUnit(data.range_days, "day"),
                 units: formatNumber(grandTotal),
               })}
             </div>