fix(feed): stop search-box flicker (keepPreviousData + debounced query)

Typing in the search box changed the feed/count/facets query keys on every keystroke, so
each query dropped to its loading state and blanked the content — the whole feed area
flickered. Debounce the search term feeding the queries (the input still updates instantly)
so they only re-run after a pause, and keep previous results on screen during a refetch via
placeholderData: keepPreviousData, so the feed and tag counts update in place without blanking.

frontend/src/lib/useDebounced.ts

@@ -0,0 +1,13 @@
+import { useEffect, useState } from "react";
+
+// Returns `value` after it has stopped changing for `delay` ms. Lets a text input stay
+// responsive (it owns the immediate value) while the query it drives only re-runs once the
+// user pauses — so we don't refetch the feed on every keystroke.
+export function useDebounced<T>(value: T, delay = 300): T {
+  const [debounced, setDebounced] = useState(value);
+  useEffect(() => {
+    const id = window.setTimeout(() => setDebounced(value), delay);
+    return () => window.clearTimeout(id);
+  }, [value, delay]);
+  return debounced;
+}