Render one chapter format, not "Ch Chapter 250"

The userscript writes last_chapter/latest_chapter as "Chapter N" and the
poller writes the same label, so the templates' own "Ch " prefix doubled it on
every real row. A manual edit through the web UI stores a bare "250", so the
list carried two formats at once and stopped being scannable.

Move the formatting out of the templates into DisplayChapter/DisplayLatest on
Bookmark, which strip a leading "Chapter"/"Ch." lead-in and re-add exactly one
"Ch ", falling back to the numeric column when the label is empty.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/templates/app.html

@@ -75,7 +75,7 @@
           {{else if .Favorite}}<span class="foot-rule brass"></span>{{end}}
         </span>
         <span class="recent-title">{{.Title}}</span>
-        <span class="recent-chapter">Ch {{.LastChapter}}{{if .HasNewChapter}} · New{{end}}</span>
+        <span class="recent-chapter">{{.DisplayChapter}}{{if .HasNewChapter}} · New{{end}}</span>
       </a>
       {{end}}
     </div>