fix: resolve image upload ERR_ACCESS_DENIED and dialog a11y warning

Eagerly read file data into memory via arrayBuffer() before constructing
FormData for the upload fetch. Clipboard-paste File objects from the MDX
editor reference transient browser data that may be discarded after the
paste event handler returns; streaming that stale reference in the fetch
body causes net::ERR_ACCESS_DENIED. Reading into an ArrayBuffer first
makes the FormData self-contained.

Also suppress the Radix UI "Missing Description" console warning on the
NewIssueDialog by setting aria-describedby={undefined}.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/NewIssueDialog.tsx

@@ -237,6 +237,7 @@ export function NewIssueDialog() {
     >
       <DialogContent
         showCloseButton={false}
+        aria-describedby={undefined}
         className={cn(
           "p-0 gap-0 flex flex-col max-h-[calc(100vh-6rem)]",
           expanded