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/DiagnosticsSettings.tsx

@@ -11,6 +11,7 @@ import SegmentedToggle from "./ui/SegmentedToggle";
 import { Section, SettingRow } from "./ui/form";
 import MyLogViewer from "./MyLogViewer";
 import { RecorderSettings } from "./DebugRecorder";
+import { formatUnit } from "../lib/format";
 
 const LEVELS: readonly DiagnosticsLevel[] = ["INFO", "DEBUG", "TRACE"];
 
@@ -171,8 +172,8 @@ export default function DiagnosticsSettings() {
                   level: levelName(current),
                   time:
                     remaining.h > 0
-                      ? t("settings.diagnostics.hoursMinutes", remaining)
-                      : t("settings.diagnostics.minutes", remaining),
+                      ? `${formatUnit(remaining.h, "hour")} ${formatUnit(remaining.m, "minute")}`
+                      : formatUnit(remaining.m, "minute"),
                 })}
               </span>
             </span>