feat(base): make column headers sticky in standalone and inline contexts

Split the unified .gridWrapper into a sticky band (containing column
headers, plus banner + toolbar in inline) and a body grid that owns
horizontal scroll. The band's vertical sticky anchor is automatic CSS:
the table's scrollport in standalone, the page in inline. A small
useHorizontalScrollSync hook mirrors body scrollLeft onto the header
and turns wheel-on-header into pan-on-body.

apps/client/src/features/editor/components/base-embed/base-embed-view.tsx

@@ -32,10 +32,12 @@ function applyExtension(wrapper: HTMLDivElement) {
   // column into empty space — same behaviour as Notion, gives the
   // table breathing room on the right when scrolled fully right.
   wrapper.style.setProperty("--embed-grid-pad-right", `${extendRight}px`);
-  // Drop the standalone "panel" frame: inline databases read as part
-  // of the document, with only cell separators as gridlines.
-  wrapper.style.setProperty("--grid-outer-border", "none");
-  wrapper.style.setProperty("--grid-outer-radius", "0");
+  // Inline sticky band clears the fixed PageHeader. Standalone leaves
+  // the var unset (resolves to the rule default of 0).
+  wrapper.style.setProperty(
+    "--sticky-band-top",
+    "var(--page-header-height)",
+  );
 }
 
 export function BaseEmbedView({ node }: NodeViewProps) {