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>

backend/web.go

@@ -6,6 +6,7 @@ import (
 	"html/template"
 	"io/fs"
 	"log"
+	"math"
 	"net/http"
 	"strconv"
 	"strings"
@@ -266,7 +267,7 @@ func (h *webHandler) uiChapter(w http.ResponseWriter, r *http.Request) {
 	}
 	raw := strings.TrimSpace(r.PostFormValue("chapter"))
 	num, err := strconv.ParseFloat(raw, 64)
-	if err != nil || num < 0 {
+	if err != nil || num < 0 || math.IsNaN(num) || math.IsInf(num, 0) {
 		http.Error(w, "chapter must be a non-negative number", http.StatusBadRequest)
 		return
 	}

backend/web_test.go

@@ -281,6 +281,42 @@ func TestFavoriteTogglesWithoutReordering(t *testing.T) {
 	}
 }
 
+// TestCardHxTargetIsValidSelectorForColonKey asserts the rendered card's
+// hx-target attributes use the fixed-string attribute-selector form
+// ([id='card-<key>']) rather than a bare CSS id-selector (#card-<key>).
+//
+// A key like "asura:solo" makes "#card-asura:solo" an invalid CSS selector:
+// the browser parses ":solo" as an unrecognised pseudo-class and htmx's
+// querySelectorAll throws SyntaxError, so the button never resolves its
+// swap target. httptest never executes htmx, so this only checks the
+// rendered attribute's shape — it is not proof the browser accepts the
+// selector, just a regression guard against reintroducing the bare-id form.
+func TestCardHxTargetIsValidSelectorForColonKey(t *testing.T) {
+	cfg := webConfig()
+	srv, store := newWebTestServer(t, cfg)
+	seed(t, store, Bookmark{
+		Key: "asura:solo", Site: "asura", SeriesID: "solo",
+		Title: "Solo Leveling", LastChapter: "45", LastChapterNum: 45,
+		UpdatedAt: 1_000_000,
+	})
+
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, uiRequest(t, cfg, http.MethodGet, "/ui/list", nil))
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	body := rr.Body.String()
+
+	want := `hx-target="[id='card-asura:solo']"`
+	if strings.Count(body, want) != 3 {
+		t.Fatalf("body has %d occurrences of %s, want 3 (favorite button, delete button, chapter form)",
+			strings.Count(body, want), want)
+	}
+	if strings.Contains(body, `hx-target="#card-asura:solo"`) {
+		t.Fatal("body still uses the bare id CSS selector, which is invalid for a key containing ':'")
+	}
+}
+
 func TestChapterOverrideMovesUpdatedAt(t *testing.T) {
 	cfg := webConfig()
 	srv, store := newWebTestServer(t, cfg)
@@ -324,7 +360,7 @@ func TestChapterOverrideRejectsBadInput(t *testing.T) {
 		Title: "Solo Leveling", LastChapterNum: 45, UpdatedAt: 1_000_000,
 	})
 
-	for _, bad := range []string{"", "abc", "-3"} {
+	for _, bad := range []string{"", "abc", "-3", "NaN", "Infinity", "-Inf"} {
 		t.Run("input "+bad, func(t *testing.T) {
 			rr := httptest.NewRecorder()
 			srv.ServeHTTP(rr, uiRequest(t, cfg, http.MethodPost,