Fix iOS markdown style metadata warnings

Resolve markdown UITextView styles to plain React Native text styles before crossing the third-party boundary. react-native-uitextview flattens parent and child styles internally, so Unistyles metadata must not be carried into that handoff.

packages/app/src/components/markdown-text-style.ts

@@ -0,0 +1,25 @@
+import { StyleSheet, type StyleProp, type TextStyle } from "react-native";
+
+export function resolvePlainMarkdownTextStyle(style: StyleProp<TextStyle>): TextStyle {
+  return stripUnistylesMetadata(StyleSheet.flatten(style) ?? {});
+}
+
+function stripUnistylesMetadata(style: TextStyle): TextStyle {
+  // iOS markdown text goes through react-native-uitextview. That library
+  // inherits text styles by flattening [parentStyle, childStyle] before handing
+  // the result to native View-backed components. If both entries are Unistyles
+  // styles, flattening preserves both `unistyles_*` metadata keys in one object,
+  // and Unistyles correctly warns that the style should have stayed array-shaped.
+  //
+  // `UITextView` is a third-party boundary, not a Unistyles-tracked component in
+  // our ownership model. Resolve the concrete style values before crossing that
+  // boundary and drop only Unistyles' private tracking metadata. This preserves
+  // iOS paragraph/spanning text selection while avoiding the metadata merge.
+  const plainStyle: Record<string, unknown> = { ...style };
+  for (const key of Object.keys(plainStyle)) {
+    if (key.startsWith("unistyles_")) {
+      delete plainStyle[key];
+    }
+  }
+  return plainStyle as TextStyle;
+}