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/book_detail.templ

@@ -3,6 +3,8 @@ package templates
 import (
 	"bookhoard/internal/handlers"
 	"fmt"
+	"net/url"
+
 	"github.com/microcosm-cc/bluemonday"
 	"strings"
 )
@@ -119,15 +121,16 @@ templ BookDetail(user User, book handlers.MediaDetail, errorMessage string) {
 						<!-- Series Badge -->
 						if book.Series.Valid && book.Series.String != "" {
 							<div class="mb-4">
-								<span
-									class="px-3 py-1 rounded-full text-sm font-semibold"
-									style="background-color: var(--accent); color: white;"
+								<a
+									href={ "/bookshelf?series_filter=" + url.QueryEscape(book.Series.String) + "&sort=series" }
+									class="px-3 py-1 rounded-full text-sm font-semibold inline-block hover:opacity-80 transition-opacity"
+									style="background-color: var(--accent); color: white; text-decoration: none;"
 								>
 									{ book.Series.String }
 									if book.SeriesNumber.Valid && book.SeriesNumber.Int32 > 0 {
 										#{ book.SeriesNumber.Int32 }
 									}
-								</span>
+								</a>
 							</div>
 						}
 						<!-- Reading Direction Badge (for manga/comics) -->