fix(app): native iOS text selection in assistant messages via UITextView

Fixes #238
Fixes #648
Refs #21

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

@@ -0,0 +1,45 @@
+import { useMemo, type ReactNode } from "react";
+import type { StyleProp, TextStyle, ViewStyle } from "react-native";
+import { UITextView } from "react-native-uitextview";
+
+interface MarkdownTextSpanProps {
+  style?: StyleProp<TextStyle>;
+  children: ReactNode;
+}
+
+// Inline span backed by UITextView so iOS gets native word-selection handles.
+// Used inside MarkdownParagraphView (which is also a UITextView on iOS); the
+// library's TextAncestorContext hoists these into UITextViewChild nodes so
+// selection drags can cross sibling spans (e.g. plain text → **bold** → code).
+export function MarkdownTextSpan({ style, children }: MarkdownTextSpanProps) {
+  return (
+    <UITextView uiTextView selectable style={style}>
+      {children}
+    </UITextView>
+  );
+}
+
+interface MarkdownParagraphViewProps {
+  paragraphStyle: ViewStyle;
+  children: ReactNode;
+}
+
+const MARKDOWN_PARAGRAPH_RESET: ViewStyle = { marginBottom: 0 };
+
+// iOS-only: paragraph wraps in UITextView so the entire paragraph is one
+// native text view. That's what unlocks cross-inline drag selection — handles
+// can span every MarkdownTextSpan child inside this paragraph.
+// ViewStyle is structurally compatible with the layout props paragraphs use
+// (margin, padding, alignment); the cast lets the existing paragraphStyle
+// flow through unchanged.
+export function MarkdownParagraphView({ paragraphStyle, children }: MarkdownParagraphViewProps) {
+  const style = useMemo(
+    () => [paragraphStyle, MARKDOWN_PARAGRAPH_RESET] as StyleProp<TextStyle>,
+    [paragraphStyle],
+  );
+  return (
+    <UITextView uiTextView selectable style={style}>
+      {children}
+    </UITextView>
+  );
+}