feat(filters): dynamic faceted chips driven by /api/facets

Topic and language chips now show live channel counts for the current filter
context instead of the static global count, and chips that match nothing are
hidden (selected chips stay so they can be cleared). Selecting a channel (or any
filter) drops the now-irrelevant chips and updates the rest. Extract a shared
filterParams() so the feed and facets queries see identical filters; the facets
query is keyed on filters so it refetches as they change. Trilingual empty-state
string when a category has no matching tags.

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

@@ -24,6 +24,7 @@
   "to": "To",
   "clearDates": "clear dates",
   "reshuffle": "Reshuffle",
+  "noMatchingTags": "No matching tags here",
   "widget": {
     "show": "Show",
     "sort": "Sort",