fix(header): show sync activity only while a job is actually running

The header's sync indicator spun "fetching history" whenever any channel
still lacked full history — i.e. perpetually, even when the scheduler was
idle between its periodic runs. It now spins only while a channel-sync job
(backfill or RSS poll) is actually running; otherwise pending deep-history
work shows as the calm, static "N without full history" link, and recent
work queued for the next run shows a static "N queued". This makes the
header coherent with the Scheduler dashboard's live state.

Backend exposes running_job_ids() from the scheduler activity and a derived
sync_active flag on /api/sync/my-status.

frontend/src/i18n/locales/hu/header.json

@@ -31,6 +31,8 @@
     "paused": "szüneteltetve",
     "syncing": "{{count}} szinkronizálás alatt",
     "backfillingHistory": "előzmény letöltése",
+    "working": "szinkronizálás…",
+    "recentQueued": "{{count}} sorban",
     "allSynced": "minden szinkronban",
     "withoutFullHistory": "{{count}} teljes előzmény nélkül",
     "fullHistoryTooltip": "Néhány csatornádnak csak a legutóbbi feltöltései vannak meg. Kattints a csatornakezelő megnyitásához (ezekre szűrve), és kérd le a teljes archívumukat.",