feat(feed): removable active-filter chips above the feed

With the filter panel now collapsed to a tab, there was no way to see what was
narrowing the feed without opening it. Every applied filter is now a chip you
can drop on its own, with a Clear all when more than one is on.

The panel's count badge and the chip row read the SAME list (lib/useActiveFilters),
so they can't disagree about what's applied; that list also owns the reset both
"Clear all"s share. Replaces the hand-summed activeCount in Sidebar.

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

@@ -1,4 +1,11 @@
 {
+  "chips": {
+    "label": "Filtered by",
+    "remove": "Remove filter: {{name}}",
+    "contentNone": "No content types",
+    "maxAge_one": "Last {{count}} day",
+    "maxAge_other": "Last {{count}} days"
+  },
   "loading": "Loading feed…",
   "loadError": "Couldn't load the feed.",
   "emptyTitle": "Your feed is empty",