fix(chat): keep large tables responsive on iOS

Selectable table cells each installed a window-level gesture recognizer, causing every touch to fan out across the entire table. Keep prose selectable while rendering iOS table text without selection.

packages/app/src/components/markdown-text-selection.tsx

@@ -0,0 +1,17 @@
+import { createContext, type ReactNode, useContext } from "react";
+
+export type MarkdownTextSurface = "prose" | "table-cell";
+
+const MarkdownTextSurfaceContext = createContext<MarkdownTextSurface>("prose");
+
+export function MarkdownTableCellText({ children }: { children: ReactNode }) {
+  return <MarkdownTextSurfaceContext value="table-cell">{children}</MarkdownTextSurfaceContext>;
+}
+
+export function useMarkdownTextSurface(): MarkdownTextSurface {
+  return useContext(MarkdownTextSurfaceContext);
+}
+
+export function iosMarkdownTextIsSelectable(surface: MarkdownTextSurface): boolean {
+  return surface !== "table-cell";
+}