refactor(channels): collapse the stats row into a Stats hover chip (R13 S3)

The six inline catalog stats crammed into the tab row become one compact
"Stats" chip that reveals the numbers on hover — a new ChannelStatsPopover on
the AnchoredPopover hover mode, one icon-per-row (channels, recent synced, full
history, [ETA], my videos, quota left). Kept in the manager's own toolbar (the
header SyncStatus is global; these are channel-catalog figures). Retires the
local Stat helper + the now-unused formatEta import. i18n channels.stats.label
(HU+EN). Self-E2E'd: hover reveals the list. gate green (tsc/eslint-0/prettier).

frontend/src/i18n/locales/en/channels.json

@@ -77,6 +77,7 @@
     "actions": "Actions"
   },
   "stats": {
+    "label": "Stats",
     "channels": "Channels",
     "channelsHint": "Channels you're subscribed to.",
     "recentSynced": "Recent synced",

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

@@ -76,6 +76,7 @@
     "actions": "Műveletek"
   },
   "stats": {
+    "label": "Statisztika",
     "channels": "Csatorna",
     "channelsHint": "Csatornák, amelyekre feliratkoztál.",
     "recentSynced": "Legutóbbi szinkronizálva",