fix(builder): duplicate inserts+selects after source; select-parent/styles-sheet/canvas-pad mobile fixes

Ship-blocking fix (Fable consult): useNodeActions.duplicate() appended the
regenerated tree at the end of the parent while leaving the ORIGINAL
selected, so duplicating a top-level section landed the copy off-screen at
the bottom of the page with no visible change -- shared by both the mobile
selection toolbar and the desktop right-click ContextMenu. Now inserts the
copy immediately after the source (actions.addNodeTree(tree, parentId,
sourceIndex + 1)) and selects it (actions.selectNode(tree.rootNodeId)),
falling back to append-at-end if the source's index can't be resolved.
Mobile also scrolls the new node into view.

Three cheap fast-follows:
- canSelectParent on useNodeActions (false when the node's parent is ROOT
  or missing); MobileSelectionToolbar disables "Select Parent" instead of
  dead-ending on a page-wide ROOT outline with no toolbar of its own.
- Opening the Styles sheet on mobile now scrolls the selected node above
  the 65dvh sheet; a temporary generous bottom-padding class handles the
  case where the node is the last thing on the page and there'd otherwise
  be no room left to scroll it into view.
- Canvas gets bottom padding equal to the fixed selection toolbar's height
  while it's visible, so the last section of a short page isn't stuck
  permanently underneath it.

Desktop duplicate behavior improves (inserts after + selects) via the
shared hook; no toHtml changes. Verified live via Playwright at 375px and
1280px (screenshots in craft/scratchpad/mobileB2/).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/styles/editor.css

@@ -34,6 +34,11 @@
   /* Phase A (mobile): fixed bottom tab bar height, used to size
      .editor-container and the safe-area padding math below. */
   --mobile-tabbar-height: 58px;
+  /* Phase B fast-follow item 4: rendered height of `.mobile-selection-toolbar`
+     (44px min-height content + 4px top/bottom padding + 1px border) -- used
+     to pad the canvas scroll area clear of it while it's visible, see
+     `.editor-canvas.has-mobile-selection-toolbar` below. */
+  --mobile-selection-toolbar-height: 53px;
 
   /* Mobile-A2: explicit overlay layer scale (was ad-hoc numbers scattered
      across the mobile block + Modal.tsx, causing accidental stacking --
@@ -1685,6 +1690,31 @@ body {
     background: rgba(239, 68, 68, 0.15);
   }
 
+  /* Fast-follow item 4: while the toolbar above is visible, give the canvas
+     scroll area enough extra bottom padding to clear it (on top of the
+     regular 12px `.editor-canvas` padding, not replacing it), so the last
+     section of a short page is still reachable by scrolling instead of
+     sitting permanently underneath the fixed toolbar. */
+  .editor-canvas.has-mobile-selection-toolbar {
+    padding-bottom: calc(12px + var(--mobile-selection-toolbar-height));
+  }
+
+  /* Fast-follow item 3: while the Styles sheet is open with a node selected,
+     `MobileSelectionToolbar`'s scroll-into-view effect needs somewhere to
+     scroll THE NODE TO even when it's the last thing on the page -- without
+     extra room below it, the canvas is already at its max scrollTop and the
+     node's top can't be brought up past wherever it naturally landed
+     (verified live: the last section stayed stuck under the sheet). A full
+     extra viewport's worth of bottom padding guarantees any node's top can
+     always reach the very top of the canvas. Temporary/cosmetic cost (a
+     stretch of blank canvas below the real content while this is active) is
+     an acceptable trade for the node never being permanently unreachable.
+     -------------------------------------------------------------------- */
+  .editor-canvas.has-mobile-styles-sheet {
+    padding-bottom: 100vh;
+    padding-bottom: 100dvh;
+  }
+
   /* --------------------------------------------------------------------
      Bottom sheet (BottomSheet.tsx)
      -------------------------------------------------------------------- */