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/channelColumns.tsx

@@ -1,7 +1,7 @@
 import type { ComponentProps, ReactNode } from "react";
 import type { TFunction } from "i18next";
 import { NO_TAG_SENTINEL, type ManagedChannel } from "../lib/api";
-import { formatCountOrDash, formatTotalHours, relativeTime } from "../lib/format";
+import { formatCountOrDash, formatTotalHours } from "../lib/format";
 import type { Column } from "./ui/DataTable";
 import { PriorityStepper } from "./ui/PriorityStepper";
 import ChannelLink from "./ChannelLink";
@@ -9,6 +9,7 @@ import AboutCell from "./AboutCell";
 import { ChannelTagsControl, type SuggestedTag } from "./ChannelTags";
 import { StoredCell } from "./channelCells";
 import { isFullySynced } from "./channelSync";
+import { RelativeTime } from "./ui/RelativeTime";
 
 /** The shared "subscribers" DataTable column (right-aligned, sortable, em-dash for null) — identical
  * in the Channels manager and the ChannelDiscovery table, which show subscriber counts on different
@@ -148,7 +149,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       width: "6rem",
       sortable: true,
       sortValue: (c) => (c.last_video_at ? new Date(c.last_video_at).getTime() : 0),
-      render: (c) => <span className="text-muted">{relativeTime(c.last_video_at) || "—"}</span>,
+      render: (c) => <RelativeTime at={c.last_video_at} empty="—" className="text-muted" />,
     },
     {
       key: "length",