refactor(ui): unify Channels + Stats filter chips (R13 S2c step 4)

- channel-manager sync-status chips (F2) -> FilterChipGroup (rounded-lg -> the
  canonical rounded-full), controls-row alignment preserved via className
- channel-manager user-tag chips (multi-select) restyled to filterChipClass
  (rounded-lg -> rounded-full) so the whole manager chip set is one look
- stats system time-range chips (F4) -> FilterChipGroup (text-sm -> canonical)

Adds aria-label i18n (channels.filters.syncLabel, stats.rangeLabel; HU+EN).
Self-E2E'd: sync chips + tag chips rounded-full, view TABS stay rounded-lg
(their own canonical), stats 7/30/90d toggles. gate green (tsc/eslint-0/
vitest 136/knip/prettier). Branch not merged.

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

@@ -39,6 +39,7 @@
     }
   },
   "filters": {
+    "syncLabel": "Sync status",
     "all": "All",
     "needsFull": "Needs full history",
     "fullySynced": "Fully synced"

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

@@ -5,6 +5,7 @@
   },
   "title": "API quota usage",
   "rangeDays": "{{count}}d",
+  "rangeLabel": "Time range",
   "introBefore": "YouTube Data API units attributed by who triggered the spend. ",
   "introSystem": "System",
   "introAfter": " is shared background work (scheduled backfill, enrichment, resync) that isn't tied to one person.",

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

@@ -39,6 +39,7 @@
     }
   },
   "filters": {
+    "syncLabel": "Szinkron állapot",
     "all": "Mind",
     "needsFull": "Hiányos előzmény",
     "fullySynced": "Teljesen szinkronizálva"

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

@@ -5,6 +5,7 @@
   },
   "title": "API-kvóta használat",
   "rangeDays": "{{count}} nap",
+  "rangeLabel": "Időtartomány",
   "introBefore": "A YouTube Data API-egységek aszerint, hogy ki váltotta ki a felhasználást. A ",
   "introSystem": "Rendszer",
   "introAfter": " a megosztott háttérmunka (ütemezett letöltés, gazdagítás, újraszinkronizálás), amely nem köthető egy személyhez.",