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/store_test.go

@@ -1034,3 +1034,41 @@ func TestOpenStoreMigratesAsuraHashlessCollision(t *testing.T) {
 		t.Fatalf("merge kept wrong row: %+v", merged)
 	}
 }
+
+func TestDisplayChapter(t *testing.T) {
+	cases := []struct {
+		name string
+		raw  string
+		num  float64
+		want string
+	}{
+		{"userscript label", "Chapter 250", 250, "Ch 250"},
+		{"abbreviated with dot", "Ch. 12", 12, "Ch 12"},
+		{"lowercase abbreviation", "ch 3", 3, "Ch 3"},
+		{"bare number from a manual edit", "45.5", 45.5, "Ch 45.5"},
+		{"empty falls back to the number", "", 7, "Ch 7"},
+		{"empty and zero", "", 0, "Ch 0"},
+		{"label with no number passes through", "Prologue", 0, "Prologue"},
+		{"title-like label keeps its words", "Chapter of the Sun", 0, "of the Sun"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := displayChapter(tc.raw, tc.num); got != tc.want {
+				t.Errorf("displayChapter(%q, %v) = %q, want %q", tc.raw, tc.num, got, tc.want)
+			}
+		})
+	}
+
+	b := Bookmark{LastChapter: "Chapter 9", LastChapterNum: 9}
+	if got := b.DisplayChapter(); got != "Ch 9" {
+		t.Errorf("DisplayChapter() = %q, want %q", got, "Ch 9")
+	}
+	if got := b.DisplayLatest(); got != "Ch 0" {
+		t.Errorf("DisplayLatest() with nothing captured = %q, want %q", got, "Ch 0")
+	}
+	num := 11.0
+	b.LatestChapter, b.LatestChapterNum = "Chapter 11", &num
+	if got := b.DisplayLatest(); got != "Ch 11" {
+		t.Errorf("DisplayLatest() = %q, want %q", got, "Ch 11")
+	}
+}