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.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+import { iosMarkdownTextIsSelectable } from "./markdown-text-selection";
+
+describe("markdown text selection", () => {
+  it("uses plain text only for iOS table cells", () => {
+    expect({
+      tableCell: iosMarkdownTextIsSelectable("table-cell"),
+      prose: iosMarkdownTextIsSelectable("prose"),
+    }).toEqual({
+      tableCell: false,
+      prose: true,
+    });
+  });
+});