Improve toolbar positioning to avoid keyboard obstruction

Update toolbar positioning logic to flip above the editor when necessary to prevent it from being hidden by the visual viewport or the software keyboard, addressing issue #581.

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artifacts/tx-os/src/components/editable-cell.tsx

@@ -545,9 +545,11 @@ function FormattingToolbar({
       : "bg-white text-[#0B1E3F] hover:bg-gray-100 border border-gray-200");
 
   // Position the toolbar via a portal so it escapes table cells with
-  // overflow:hidden. We try to place it above the editor, but flip below
-  // when there isn't enough room (e.g. first row of the table). We also
-  // clamp horizontally to keep it inside the viewport.
+  // overflow:hidden. Default placement is BELOW the editor; we only
+  // flip ABOVE when staying below would hide the toolbar under the iOS
+  // soft keyboard or off the bottom of the visible viewport (#581).
+  // We also clamp horizontally to keep it inside the table's scroll
+  // container (falling back to the viewport).
   const [pos, setPos] = useState<{ top: number; left: number } | null>(null);
   const [toolbarEl, setToolbarEl] = useState<HTMLDivElement | null>(null);
   const setToolbarNode = useCallback(
@@ -595,7 +597,7 @@ function FormattingToolbar({
       // swatches / Save / Cancel become unreachable on iPad. See #581.
       const vvTop = vv.offsetTop;
       const vvBottom =
-        vv.offsetTop + (vv.height || window.innerHeight);
+        vv.offsetTop + (vv.height ?? window.innerHeight);
       let top = rect.bottom + 4;
       if (top + tbHeight > vvBottom - 4) {
         const above = rect.top - tbHeight - 4;