feat(series): add series browse template, nav link, and clickable badge

Create templates/series.templ with:
- Library selector dropdown (sticky, same pattern as dashboard)
- Loading spinner overlay for AJAX library switching
- Series grid with stacked-cascade multi-cover cards
- Empty state when no series found
- Pagination with Previous/Next links
- SeriesCard sub-template linking to filtered bookshelf view

Add 'Series' nav link in header between 'All Books' and 'Collections'.

Make series badge on book detail page clickable, linking to
/bookshelf?series_filter=<name>&sort=series.

Add 'Continue Series' option to restore system collection modal.

templates/restore_system_collection_modal.templ

@@ -19,6 +19,7 @@ templ RestoreSystemCollectionModal() {
 						<option value="Recently Added">Recently Added</option>
 						<option value="Recently Read">Recently Read</option>
 						<option value="Not Started">Not Started</option>
+						<option value="Continue Series">Continue Series</option>
 					</select>
 					<div class="flex justify-end space-x-3 mt-6">
 						<button type="button" @click="closeCollectionModal()" class="btn-secondary px-4 py-2 rounded-lg">Cancel</button>