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/hu/notifications.json

@@ -11,11 +11,5 @@
   "openInManager": "Csatornakezelő",
   "openOnYouTube": "Megnyitás a YouTube-on",
   "unhidden": "Megjelenítve: „{{title}}”",
-  "unwatched": "Megtekintés visszavonva: „{{title}}”",
-  "time": {
-    "justNow": "az imént",
-    "minutes": "{{count}} perce",
-    "hours": "{{count}} órája",
-    "days": "{{count}} napja"
-  }
+  "unwatched": "Megtekintés visszavonva: „{{title}}”"
 }