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/components/Sidebar.tsx

@@ -7,6 +7,7 @@ import { defaultLayout, type PanelLayout } from "../lib/panelLayout";
 import { shareUrl } from "../lib/urlState";
 import { notify } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
+import { clearedFilters, useActiveFilters } from "../lib/useActiveFilters";
 import TagManager from "./TagManager";
 import SavedViewsWidget from "./SavedViewsWidget";
 import SidePanel from "./SidePanel";
@@ -21,18 +22,6 @@ const DATE_PRESETS: { days: number; key: string }[] = [
   { days: 365, key: "1year" },
 ];
 
-// Filter values owned by the sidebar (everything except the header search `q` and the
-// `scope` mode, both preserved across a "Clear all").
-const DEFAULT_SIDEBAR_FILTERS: Omit<FeedFilters, "q" | "scope"> = {
-  tags: [],
-  tagMode: "or",
-  sort: "newest",
-  includeNormal: true,
-  includeShorts: false,
-  includeLive: false,
-  show: "unwatched",
-};
-
 function TagChip({
   tag,
   count,
@@ -143,21 +132,9 @@ export default function Sidebar({
     });
   }
 
-  const dateActive = !!filters.maxAgeDays || !!filters.dateFrom || !!filters.dateTo;
-  const contentChanged =
-    !filters.includeNormal || filters.includeShorts || filters.includeLive;
-  const scopeActive = filters.scope === "all";
-  const sourceActive =
-    filters.scope === "all" && !!filters.librarySource && filters.librarySource !== "organic";
-  const activeCount =
-    filters.tags.length +
-    (filters.show !== "unwatched" ? 1 : 0) +
-    (filters.sort !== "newest" && filters.sort !== "relevance" ? 1 : 0) +
-    (contentChanged ? 1 : 0) +
-    (filters.channelId ? 1 : 0) +
-    (dateActive ? 1 : 0) +
-    (scopeActive ? 1 : 0) +
-    (sourceActive ? 1 : 0);
+  // The badge counts exactly what the chip row above the feed shows — same list, so they can't
+  // disagree about what's applied.
+  const activeCount = useActiveFilters(filters, setFilters).length;
   const active = activeCount > 0;
 
   async function shareView() {
@@ -171,7 +148,7 @@ export default function Sidebar({
 
   function clearAll() {
     setCustomDates(false);
-    setFilters({ ...DEFAULT_SIDEBAR_FILTERS, q: filters.q, scope: "my" });
+    setFilters(clearedFilters(filters));
   }
 
   const available: Record<string, boolean> = {