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.

web/src/book-detail.ts

@@ -311,6 +311,23 @@ Alpine.data("bookDetail", () => {
         showToast("Could not delete the note", "error");
       }
     },
+    async deleteBannerHighlight(row: AnnotationRow) {
+      if (row.kind !== "highlight") return;
+      const mediaId = getMediaId();
+      const hlId = row.key.slice("hl-".length);
+      try {
+        const resp = await fetch(`/api/media-items/${mediaId}/highlights/${hlId}`, {
+          method: "DELETE",
+          headers: { Authorization: getAuthHeader() },
+        });
+        if (!resp.ok) throw new Error(String(resp.status));
+        // Tombstoned server-side: restorable from the Deleted modal.
+        showToast("Highlight deleted — restorable from Deleted", "success");
+        await this.fetchBannerAnnotations();
+      } catch (_e) {
+        showToast("Could not delete the highlight", "error");
+      }
+    },
     showExportDialog(): void {
       if (this.bannerRows.length === 0) return;
       this.annotationsExportOpen = true;