fix(backend): valid hx-target selector and reject NaN/Infinity chapter input

hx-target="#card-<key>" is an invalid CSS selector for any key containing
a colon (every real bookmark key is "<site>:<series_id>"), so htmx threw
before swapping and the favourite/delete/chapter-override controls were
dead in the browser. Switch to the attribute-selector form
[id='card-<key>'], which querySelectorAll accepts regardless of the id's
characters.

Also close a validation gap in uiChapter: strconv.ParseFloat accepts
"NaN"/"Infinity"/"-Inf" with err == nil, and every comparison against NaN
is false, so num < 0 let both through to last_chapter_num and permanently
broke HasNewChapter. Reject non-finite values explicitly.

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

backend/templates/card.html

@@ -15,19 +15,19 @@
       <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">
+              hx-target="[id='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-target="[id='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">
+          hx-target="[id='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>