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

templates/bookshelf.templ

@@ -93,7 +93,7 @@ templ BookShelf(
 								<datalist id="author-datalist"></datalist>
 							</div>
 							<!-- Tags Filter with Autocomplete -->
-							<div class="flex-1 min-w-[150px]">
+							<div class="flex-1 min-w-[150px] relative">
 								<label class="block text-sm font-medium mb-2" style="color: var(--text-secondary)">Tags</label>
 								<input
 									type="text"
@@ -101,10 +101,29 @@ templ BookShelf(
 									placeholder="Filter by tags"
 									class="w-full px-3 py-2 border rounded-lg"
 									style="background-color: var(--bg-primary); color: var(--text-primary); border-color: var(--border);"
-									list="tags-datalist"
 									@input.debounce.300ms="if($el.value.length >= 2) fetchTagValues($el)"
+									@keydown="onTagFilterKeydown($event)"
+									@blur="hideTagDropdown()"
 								/>
-								<datalist id="tags-datalist"></datalist>
+								<div
+									x-show="showTagDropdown"
+									x-transition
+									class="absolute z-50 mt-1 w-full rounded-lg shadow-lg border max-h-48 overflow-y-auto"
+									style="background-color: var(--bg-primary); border-color: var(--border);"
+								>
+									<template x-for="sug in tagSuggestions" :key="sug.value">
+										<button
+											type="button"
+											class="w-full text-left px-3 py-2 text-sm flex justify-between items-center"
+											:class="tagHighlightIndex === tagSuggestions.indexOf(sug) ? 'opacity-80' : ''"
+											:style="tagHighlightIndex === tagSuggestions.indexOf(sug) ? 'background-color: var(--bg-secondary); color: var(--text-primary);' : 'color: var(--text-primary);'"
+											@click="selectTagSuggestion(sug.value)"
+										>
+											<span x-text="sug.value"></span>
+											<span class="text-xs" style="color: var(--text-secondary);" x-text="sug.count + ' books'"></span>
+										</button>
+									</template>
+								</div>
 							</div>
 							// <!-- Genre Filter with Autocomplete -->
 							// <div class="flex-1 min-w-[150px]">