fix: make code file preview text selectable on iOS (#447)

Add `selectable` prop to the code line Text component so users can
long-press to select and copy text on iOS. On web/desktop this is
already the default behavior via CSS user-select.

Markdown file preview has a similar issue but requires a different
fix (custom rules for react-native-markdown-display), left for a
follow-up.

Refs #238
Related #21

Co-authored-by: muzhi1991 <muzhi1991@limuzhideMac-mini-2.local>

packages/app/src/components/file-pane.tsx

@@ -72,7 +72,7 @@ const CodeLine = React.memo(function CodeLine({
       <View style={[codeLineStyles.gutter, { width: gutterWidth }]}>
         <Text style={[codeLineStyles.gutterText, { color: baseColor }]}>{String(lineNumber)}</Text>
       </View>
-      <Text style={codeLineStyles.lineText}>
+      <Text selectable style={codeLineStyles.lineText}>
         {tokens.map((token, index) => (
           <Text
             key={index}