pin Back-to-Deck bar to the viewport

#deckEditBar used position: absolute, which made it a child of the
scrolling #previewShell — panning or zooming the canvas dragged
the bar off-screen. Switching to position: fixed pins it to the
viewport so it stays put at top-center regardless of canvas pan or
zoom.

src/index.html

@@ -769,7 +769,12 @@
     }
     .deckSlideActions button:hover { background: #f1f5f9; }
     #deckEditBar {
-      position: absolute; top: 10px; left: 50%; transform: translateX(-50%);
+      /* Pinned to the viewport so panning / zooming the canvas
+         doesn't drag the bar along. The bar still visually sits over
+         the canvas area because the sidebar is on the left — top:10px
+         + left:50% lands roughly centred over the canvas at common
+         widths. */
+      position: fixed; top: 10px; left: 50%; transform: translateX(-50%);
       z-index: 60; align-items: center; gap: 10px;
       background: var(--panel, #fff); border: 1px solid var(--line);
       border-radius: 999px; padding: 5px 14px 5px 8px;