feat: implement 3-state has_cover filter and filter item component

Template changes for bookshelf page:

Cover filter (tristate button):
- Replace checkbox with 3-state button: Any (null) → Has Cover → No Cover
- Add Alpine state management for hasCoverState (true/false/null)
- Button shows dynamic icon and label based on state:
  - ○ Cover: Any
  - ✓ Has Cover
  - ✗ No Cover
- Hidden input conditionally rendered by Alpine (x-if="hasCoverState !== null")
- Only submits "true"/"false" or not at all, never empty string
- Theme-aware styling using CSS variables and color-mix()

Filter management improvements:
- Add name="library" attribute to library select for proper form submission
- Create filter_item.templ component for rendering individual filter items
- Add Load Filter and Delete Filter buttons with Alpine event handlers
- Update save filter form to use HTMX attributes:
  - hx-post, hx-target, hx-swap, hx-include for AJAX submission
  - @htmx:afterRequest event for modal cleanup

This fixes issues where:
- Library wasn't being submitted with search/filter requests
- has_cover was sending empty string causing no results
- Saved filters couldn't be loaded or deleted

templates/filter_item.templ

@@ -0,0 +1,28 @@
+package templates
+
+// FilterItem renders a single saved filter item for the list
+templ FilterItem(id string, name string) {
+	<div
+		class="flex items-center justify-between p-2 rounded hover:opacity-80"
+		style="background-color: var(--bg-primary);"
+		data-filter-id={ id }
+	>
+		<button
+			data-action="load-filter"
+			@click="loadFilter($event)"
+			class="flex-1 text-left px-2 py-1 rounded"
+			style="color: var(--text-primary);"
+		>
+			{ name }
+		</button>
+		<button
+			data-action="delete-filter"
+			@click="deleteFilter($event)"
+			class="p-1 hover:opacity-70 rounded"
+			style="color: var(--text-secondary);"
+			title="Delete filter"
+		>
+			🗑
+		</button>
+	</div>
+}