book: layers panel scopes to the active book page

The layers sidebar fell through to the case-cover sections
(Back / Spine / Front) when in book mode because
templateLayerTargetsForMode() had no book branch.

It now returns a single panel labeled "Page N" keyed to the
active entry's target (`book-page:<entryId>`), so the layers
listed and the Add-layer actions are scoped to the page the user
is editing. When no page is open (Pages overview is showing) the
helper returns an empty list and renderLayerList() naturally
renders nothing — consistent with the deck-sorter view that also
hides the layers fieldset until a slide is open.

src/index.html

@@ -8727,6 +8727,18 @@
       if (isCanvasMode(state.templateMode)) return [
         { key: 'canvas', label: 'Canvas' }
       ];
+      if (state.templateMode === 'book') {
+        // Book mode: the layers panel scopes to whichever page is
+        // open in the per-page editor. No page open → no panels
+        // (Pages overview shows tiles, not layer rows).
+        const editId = state.book?.editingEntryId;
+        if (editId) {
+          const idx = (state.book?.pages || []).findIndex(p => p.id === editId);
+          const human = idx >= 0 ? `Page ${idx + 1}` : 'Current page';
+          return [{ key: `book-page:${editId}`, label: human }];
+        }
+        return [];
+      }
       // Cover mode. In combined-wrap mode the artwork is one open canvas, so
       // back/spine/front sub-regions aren't meaningful — collapse to a single
       // "Cover trim area" panel that still lists any pre-existing back/spine/