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.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"regexp"
+	"strconv"
 	"strings"
 
 	_ "modernc.org/sqlite"
@@ -41,6 +42,41 @@ func (b Bookmark) HasNewChapter() bool {
 	return b.LatestChapterNum != nil && *b.LatestChapterNum > b.LastChapterNum
 }
 
+// chapterLeadIn matches the prefix the userscript and the poller both write
+// ("Chapter 250"), so the UI can add exactly one "Ch " of its own instead of
+// doubling it. A manual edit through the web UI stores a bare "250", which is
+// the same string minus the lead-in.
+var chapterLeadIn = regexp.MustCompile(`(?i)^\s*(?:chapter|ch\.?)\s*`)
+
+func displayChapter(raw string, num float64) string {
+	rest := strings.TrimSpace(chapterLeadIn.ReplaceAllString(raw, ""))
+	if rest == "" {
+		rest = strconv.FormatFloat(num, 'f', -1, 64)
+	}
+	// "Ch " only makes sense in front of a number; anything else is a label the
+	// site gave us, so pass it through as written.
+	if rest[0] < '0' || rest[0] > '9' {
+		return rest
+	}
+	return "Ch " + rest
+}
+
+// DisplayChapter is the read-progress line: one canonical "Ch N" whatever
+// format the write came in as.
+func (b Bookmark) DisplayChapter() string {
+	return displayChapter(b.LastChapter, b.LastChapterNum)
+}
+
+// DisplayLatest is the same for the newest published chapter, which arrives
+// with the same "Chapter N" lead-in from both the userscript and the poller.
+func (b Bookmark) DisplayLatest() string {
+	var num float64
+	if b.LatestChapterNum != nil {
+		num = *b.LatestChapterNum
+	}
+	return displayChapter(b.LatestChapter, num)
+}
+
 // ContinueURL is where the Continue button points: the chapter last read, or
 // the series page when no chapter URL was ever captured.
 func (b Bookmark) ContinueURL() string {