refactor(i18n,format): translate ETA strings; consolidate time/duration formatters

- formatEta was hardcoded English (violated the trilingual rule) -> time.eta.* keys
  in EN/HU/DE; Scheduler's countdown 'now' likewise.
- relativeFromMs added to format.ts; NotificationsPanel drops its duplicate
  relativeTime/relativeFromMs and the now-orphaned notifications.time.* keys.
- Channels' fmtTotalDuration moved to format.ts as formatTotalHours.

frontend/src/components/Scheduler.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
+import i18n from "../i18n";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import {
   Activity,
@@ -31,7 +32,7 @@ function secsUntil(iso: string | null): number | null {
 }
 
 function fmtCountdown(s: number): string {
-  if (s <= 0) return "now";
+  if (s <= 0) return i18n.t("time.eta.now");
   if (s < 60) return `${s}s`;
   const m = Math.floor(s / 60);
   if (m < 60) return `${m}m ${s % 60}s`;