fix(desktop): make file-preview source + markdown selectable (#44648)

body sets user-select:none for native feel and opts text back in only via
[data-selectable-text='true']; the preview's source and rendered-markdown
panes never set it, so code couldn't be selected or copied. Tag the Shiki
code column and the markdown root. The attribute stays off the SourceView
grid root so the gutter keeps its select-none and line numbers don't bleed
into copied text.

apps/desktop/src/app/chat/right-rail/preview-file.tsx

@@ -287,7 +287,7 @@ const MARKDOWN_COMPONENTS = {
 
 function MarkdownPreview({ text }: { text: string }) {
   return (
-    <div className="preview-markdown mx-auto max-w-3xl px-4 py-3 text-sm text-foreground">
+    <div className="preview-markdown mx-auto max-w-3xl px-4 py-3 text-sm text-foreground" data-selectable-text="true">
       <Streamdown components={MARKDOWN_COMPONENTS} controls={false} mode="static" parseIncompleteMarkdown={false}>
         {text}
       </Streamdown>
@@ -383,7 +383,10 @@ function SourceView({ filePath, language, text }: { filePath: string; language:
           )
         })}
       </div>
-      <div className="relative [&_pre]:m-0 [&_pre]:px-3 [&_pre]:py-3 [&_pre]:bg-transparent!">
+      <div
+        className="relative [&_pre]:m-0 [&_pre]:px-3 [&_pre]:py-3 [&_pre]:bg-transparent!"
+        data-selectable-text="true"
+      >
         {selection && (
           <div
             aria-hidden