feat(bookshelf): replace broken datalist tag filter with custom autocomplete

The HTML <datalist> approach for tag autocomplete was unreliable across
browsers — showed empty suggestions or no dropdown at all.

Replace with a custom Alpine.js dropdown:
- New tag-dropdown.ts shared module: searchTagSuggestions() queries
  /api/media-items/search?tags=...&library_id=... and returns results
- Bookshelf: absolute-positioned dropdown below tags_filter input, shows
  tag name + book count per suggestion
- Keyboard navigation: ArrowUp/Down to highlight, Enter to select,
  Escape to close
- Click suggestion to populate the filter input

web/src/tag-dropdown.ts

@@ -0,0 +1,28 @@
+export interface TagSuggestion {
+  value: string;
+  count: number;
+}
+
+export async function searchTagSuggestions(
+  query: string,
+  libraryId: string,
+): Promise<TagSuggestion[]> {
+  if (!query || query.length < 2 || !libraryId) return [];
+
+  const token = localStorage.getItem("token");
+  if (!token) return [];
+
+  try {
+    const response = await fetch(
+      `/api/media-items/search?tags=${encodeURIComponent(query)}&library_id=${libraryId}&limit=20`,
+      { headers: { Authorization: `Bearer ${token}` } },
+    );
+
+    if (!response.ok) return [];
+
+    const data = await response.json();
+    return (data.results || []) as TagSuggestion[];
+  } catch {
+    return [];
+  }
+}