improvement(filters): count Source + Library scope as active; unify Clear all; add search clear (X)

The "Filters N active" badge and Clear all only tracked the sidebar facets
(tags/show/sort/content/date/channel), so the toolbar-level Source and the
Mine/Library scope neither bumped the count nor reset — inconsistent with
filtersToParams (which treats them as filters, so Save view/Share already
captured them).

- activeCount now includes Library scope and a non-default Source; "relevance"
  (auto-applied while searching, no manual option) is excluded from the sort
  count so typing a query no longer inflates the badge.
- Clear all resets scope back to "my" (Mine) and drops Source to organic,
  alongside the facets. The search q is intentionally preserved.
- The header search box gets a clear (X) button when it holds a value, so the
  search has its own reset (clearing it reverts the relevance sort to newest).

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

@@ -1,6 +1,7 @@
 {
   "feed": "Feed",
   "searchPlaceholder": "Search your subscriptions…",
+  "clearSearch": "Clear search",
   "searchYoutube": "Search YouTube",
   "searchYoutubeTip": "Search live on YouTube for this term (uses shared API quota). You can also press Enter.",
   "channelManager": "Channel manager",