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/i18n/locales/en/time.json

@@ -6,5 +6,14 @@
   "daysAgo": "{{count}}d ago",
   "weeksAgo": "{{count}} wk ago",
   "monthsAgo": "{{count}} mo ago",
-  "yearsAgo": "{{count}} yr ago"
+  "yearsAgo": "{{count}} yr ago",
+  "eta": {
+    "now": "now",
+    "done": "done",
+    "lessThanHour": "< 1 hour",
+    "hours_one": "~{{count}} hour",
+    "hours_other": "~{{count}} hours",
+    "days_one": "~{{count}} day",
+    "days_other": "~{{count}} days"
+  }
 }