fix(web): banner highlight delete; retention wording states the days

The book-detail banner's highlight rows gain Delete (tombstoned
server-side, restorable from the Deleted modal — same as the reader
drawers already offered). The deleted-annotations modal now says the
retention in plain language — 'kept for 30 days' from the
annotation_tombstone_ttl_days tunable (threaded through BookDetail →
NotesHighlightsModal; 1-day singular and retention-disabled variants
handled) — instead of referencing a settings name a reader cannot see.

internal/router/frontend.go

@@ -1436,7 +1436,10 @@ func registerFrontendRoutes(cfg *Config) {
 
 		// Render template
 		var buf bytes.Buffer
-		err = templates.BookDetail(user, detail, "").Render(c.Request().Context(), &buf)
+		// The deleted-annotations modal states the retention in plain days
+		// (the tunable's user-facing value), never as a settings name.
+		tombstoneDays := cfg.Settings.TombstoneTTL() / (24 * time.Hour)
+		err = templates.BookDetail(user, detail, "", int(tombstoneDays)).Render(c.Request().Context(), &buf)
 		if err != nil {
 			return err
 		}