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

Fixes #238
Fixes #648
Refs #21

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

@@ -0,0 +1,37 @@
+import { useMemo, type ReactNode } from "react";
+import { Text, View, type StyleProp, type TextStyle, type ViewStyle } from "react-native";
+
+interface MarkdownTextSpanProps {
+  style?: StyleProp<TextStyle>;
+  children: ReactNode;
+}
+
+// Android's <Text selectable> enables per-text-node selection natively. Each
+// sibling Text is its own selection scope — drag can't span across siblings
+// (that requires a single UITextView ancestor and is iOS-only).
+export function MarkdownTextSpan({ style, children }: MarkdownTextSpanProps) {
+  return (
+    <Text selectable style={style}>
+      {children}
+    </Text>
+  );
+}
+
+interface MarkdownParagraphViewProps {
+  paragraphStyle: ViewStyle;
+  children: ReactNode;
+}
+
+const MARKDOWN_PARAGRAPH_RESET: ViewStyle = { marginBottom: 0 };
+
+// Paragraph stays a <View>, not a <Text>, for layout fidelity. RN Android's
+// text engine *does* accept inline View children (TextInlineViewPlaceholderSpan
+// in ReactBaseTextShadowNode), so this isn't a crash-avoidance choice — but
+// inline-placeholder spans collapse block-level children (e.g. paragraph
+// images) into one-character placeholders, which destroys image row layout.
+// <View> preserves the original block layout; the trade-off is no cross-span
+// selection on Android (a UITextView-style trick has no Android equivalent).
+export function MarkdownParagraphView({ paragraphStyle, children }: MarkdownParagraphViewProps) {
+  const style = useMemo(() => [paragraphStyle, MARKDOWN_PARAGRAPH_RESET], [paragraphStyle]);
+  return <View style={style}>{children}</View>;
+}