fix(plex): explain an empty search when filters are also active

A Plex search that returns nothing while a collection/rating filter is
active showed a bare "No matches", hiding that the filters — not the
query — were the cause. Show how many filters are narrowing the results
and offer a one-click Clear filters inline. Translated in all three
languages.

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

@@ -57,6 +57,8 @@
   "count": "{{count}} titles",
   "searchCount": "{{count}} matches",
   "noMatches": "No matches.",
+  "noMatchesFiltered_one": "No matches — a filter is also narrowing the results.",
+  "noMatchesFiltered_other": "No matches — {{count}} filters are also narrowing the results.",
   "loading": "Loading…",
   "empty": "Nothing here yet. Run a Plex sync from the admin Config page.",
   "loadMore": "Load more",