fix library tag search: dynamic tagGroups, clear tags, cache-bust

Three bugs in library.tsx (all previously fixed, lost in UX rewrite):

1. tagGroups now recomputed from items passing current collection +
   search filters — only shows tags that will produce results
2. Collection switching clears selectedTags (prevents stale empty)
3. Cache-buster on fetch("/library.json?v=" + Date.now())

fix #71

docs/src/pages/library.tsx

@@ -76,7 +76,7 @@ export default function Library(): JSX.Element {
   const searchRef = useRef<HTMLInputElement>(null);
 
   useEffect(() => {
-    fetch("/library.json")
+    fetch("/library.json?v=" + Date.now())
       .then((r) => r.json())
       .then((d: LibraryData) => setData(d))
       .catch(() => setData({ items: [], collections: [], tags: [] }));
@@ -106,11 +106,6 @@ export default function Library(): JSX.Element {
     return () => document.removeEventListener("keydown", handler);
   }, []);
 
-  const tagGroups = useMemo(
-    () => (data ? groupTags(data.tags) : {}),
-    [data],
-  );
-
   const filtered = useMemo(() => {
     if (!data) return [];
     let items = data.items;
@@ -139,6 +134,38 @@ export default function Library(): JSX.Element {
     return items;
   }, [data, debouncedSearch, selectedCollection, selectedTags]);
 
+  // Recompute visible tags from items that pass collection + search filters
+  // so clicking a tag is guaranteed to produce results.
+  const tagGroups = useMemo(() => {
+    if (!data) return {};
+    let pool = data.items;
+    if (selectedCollection) {
+      pool = pool.filter((i) => i.collections.includes(selectedCollection));
+    }
+    if (debouncedSearch.trim()) {
+      const q = debouncedSearch.toLowerCase();
+      pool = pool.filter(
+        (i) =>
+          i.title.toLowerCase().includes(q) ||
+          i.abstract.toLowerCase().includes(q) ||
+          i.tags.some((t) => t.toLowerCase().includes(q)) ||
+          i.creators.some((c) => c.toLowerCase().includes(q)),
+      );
+    }
+    const counts: Record<string, number> = {};
+    for (const item of pool) {
+      for (const t of item.tags) {
+        counts[t] = (counts[t] || 0) + 1;
+      }
+    }
+    const liveTags: Tag[] = Object.entries(counts).map(([name, count]) => ({
+      name,
+      count,
+    }));
+    liveTags.sort((a, b) => a.name.localeCompare(b.name));
+    return groupTags(liveTags);
+  }, [data, selectedCollection, debouncedSearch]);
+
   const pageItems = useMemo(
     () => filtered.slice(page * PAGE_SIZE, (page + 1) * PAGE_SIZE),
     [filtered, page],
@@ -182,6 +209,7 @@ export default function Library(): JSX.Element {
               <button
                 onClick={() => {
                   setSelectedCollection("");
+                  setSelectedTags(new Set());
                   setPage(0);
                 }}
                 style={{
@@ -202,6 +230,7 @@ export default function Library(): JSX.Element {
                 <button
                   onClick={() => {
                     setSelectedCollection(c.key);
+                    setSelectedTags(new Set());
                     setPage(0);
                   }}
                   style={{