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/header.templ

@@ -21,6 +21,9 @@ templ Header(user User, currentPath string) {
 						>
 							All Books
 						</a>
+						<a href="/series" class="text-sm hover:opacity-80 transition-opacity" style="color: var(--text-secondary); text-decoration: none;">
+							Series
+						</a>
 						<a href="/collections" class="text-sm hover:opacity-80 transition-opacity" style="color: var(--text-secondary); text-decoration: none;">
 							Collections
 						</a>