feat(backend): favourite, chapter override, and delete fragments

Adds the three UI mutation endpoints (favourite toggle, manual chapter
override, delete) plus the card controls that call them via htmx.
Each mutation is a read-modify-write through Store.Get/Upsert so
Upsert alone decides whether updated_at moves — favouriting must not
reorder the list, only real reading progress should.

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

backend/templates/card.html

@@ -12,7 +12,26 @@
     </p>
     <div class="actions">
       <a class="primary" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer">Continue</a>
+      <button class="icon {{if .Favorite}}on{{end}}"
+              title="Favourite" aria-label="Toggle favourite"
+              hx-post="/ui/bookmarks/{{.Key}}/favorite"
+              hx-target="#card-{{.Key}}" hx-swap="outerHTML">
+        {{if .Favorite}}★{{else}}☆{{end}}
+      </button>
+      <button class="icon" title="Set chapter" aria-label="Set chapter"
+              onclick="toggleChapterForm('{{.Key}}')">✎</button>
+      <button class="icon danger" title="Remove" aria-label="Remove"
+              hx-delete="/ui/bookmarks/{{.Key}}"
+              hx-target="#card-{{.Key}}" hx-swap="outerHTML"
+              hx-confirm="Remove {{.Title}} from the list?">🗑</button>
     </div>
+    <form class="chapter-form" id="chapter-form-{{.Key}}" hidden
+          hx-post="/ui/bookmarks/{{.Key}}/chapter"
+          hx-target="#card-{{.Key}}" hx-swap="outerHTML">
+      <input name="chapter" type="number" step="0.1" min="0"
+             value="{{.LastChapterNum}}" aria-label="Chapter number" required>
+      <button type="submit">Save</button>
+    </form>
   </div>
 </article>
 {{end}}