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/card.html

@@ -21,10 +21,10 @@
       <p class="meta">
         <span class="site-{{.Site}}">{{.Site}}</span>
         <span class="sep">/</span>
-        <span class="chapter">Ch {{.LastChapter}}</span>
+        <span class="chapter">{{.DisplayChapter}}</span>
         {{if and (eq .Status "reading") .HasNewChapter}}
         <span class="sep">/</span>
-        <span class="new-chapter">Ch {{.LatestChapter}} out</span>
+        <span class="new-chapter">{{.DisplayLatest}} out</span>
         {{end}}
         {{if eq .Status "archived"}}
         <span class="sep">/</span>
@@ -86,7 +86,7 @@
         hx-post="/ui/bookmarks/{{.Key}}/chapter"
         hx-target="[id='card-{{.Key}}']" hx-swap="outerHTML"
         hx-indicator="[id='card-{{.Key}}']" hx-disabled-elt="input, button">
-    {{if .LatestChapter}}<p class="hint">Latest known: Ch {{.LatestChapter}}</p>{{end}}
+    {{if .LatestChapter}}<p class="hint">Latest known: {{.DisplayLatest}}</p>{{end}}
     <div class="field">
       {{/* max is a fat-finger guard, not a real ceiling — no series is near it. */}}
       <input name="chapter" type="number" step="0.1" min="0" max="9999"