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/SettingsPanel.tsx

@@ -1,7 +1,7 @@
 import { useState } from "react";
 import { withActiveAccount } from "../lib/activeAccount";
 import { qk } from "../lib/queryKeys";
-import { formatDateTime } from "../lib/format";
+import { formatDate, formatDateTime } from "../lib/format";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Activity, Bell, Check, Loader2, Monitor, RefreshCw, Trash2, User } from "lucide-react";
@@ -519,7 +519,7 @@ function SignInMethods({ me }: { me: Me }) {
 // and does a one-time "Plex is master" import. Admin-only (rides the server admin token) and only
 // shown when the Plex module is enabled. Two-way push/pull arrive in later phases.
 function PlexWatchSync() {
-  const { t, i18n } = useTranslation();
+  const { t } = useTranslation();
   const qc = useQueryClient();
   const link = useQuery({ queryKey: qk.plexWatchLink(), queryFn: () => api.plexWatchLink() });
   const enabled = link.data?.sync_enabled ?? false;
@@ -562,9 +562,7 @@ function PlexWatchSync() {
   // Reflect the toggle's target immediately (optimistic) so the switch responds on click rather than
   // only flipping once the multi-second import returns — otherwise it reads as "nothing happened".
   const shown = toggle.isPending ? Boolean(toggle.variables) : enabled;
-  const last = link.data?.last_watch_sync_at
-    ? new Date(link.data.last_watch_sync_at).toLocaleString(i18n.language)
-    : null;
+  const last = link.data?.last_watch_sync_at ? formatDateTime(link.data.last_watch_sync_at) : null;
 
   return (
     <Section title={t("settings.plexSync.title")}>
@@ -670,9 +668,7 @@ export function DownloadAuth() {
       ) : (
         <div className="divide-y divide-border">
           {hosts.map((h) => {
-            const when = h.updated_at
-              ? new Date(h.updated_at).toLocaleDateString(i18n.language)
-              : "";
+            const when = h.updated_at ? formatDate(h.updated_at, i18n.language) : "";
             const open = openHost === h.host_key;
             return (
               <div key={h.host_key} className="py-2" data-testid={`cookie-host-${h.host_key}`}>