book: hide cover-mode canvas panels in book mode

The cover designer's BACK / SPINE / FRONT slot placeholders +
bleed / trim / fold-line / crop-mark guides were still rendering
on the canvas-host when book mode was active, because the existing
mode-switch only hides them for customart / slide / pixelart /
deck. Add a matching `body.template-book` block that hides every
`.coverOnly` element plus the disc / jewel artifacts and the
spine-text label.

Also hides the sidebar #exportFieldset / #layersFieldset /
#printExportRow until book pages exist (matches the deck-mode and
pixelart-mode treatment — they're not in the book WORKFLOW_ORDER
anyway, but the CSS gate keeps them from sneaking in).

The canvas-host now renders empty in book mode — the Pages
overview view (A4) and Cover view (A5) fill it in subsequent
commits.

src/index.html

@@ -1467,6 +1467,25 @@
     body.template-pixelart #layersFieldset,
     body.template-pixelart #printExportRow { display: none !important; }
 
+    /* Book mode (Phase A): the canvas-host shouldn't render any of
+       the cover-mode panels (BACK / SPINE / FRONT slots, trim+bleed
+       guides, fold lines, crop marks). The Pages overview view
+       (A4) and Cover view (A5) replace this with their own DOM. The
+       sidebar export / layers panels also hide — they don't apply
+       until pages exist. */
+    body.template-book #exportFieldset,
+    body.template-book #layersFieldset,
+    body.template-book #printExportRow,
+    body.template-book .coverOnly,
+    body.template-book .diskLayer,
+    body.template-book .jewelLayer,
+    body.template-book .bleedFill,
+    body.template-book .trimArea,
+    body.template-book .foldLine,
+    body.template-book .discSpineStrip,
+    body.template-book .jewelSpineGuide,
+    body.template-book .spineText { display: none !important; }
+
     .templateLayerPanels {
       display: grid;
       gap: 10px;