Fix markdown editor rendering and pasted-link duplication

- Restore heading/list typography in the editor: MDXEditor relies on the
  browser's default styles for <h1>/<ul> sizing, which Tailwind's Preflight
  resets. Markdown shortcuts fired but the block looked unchanged. Style the
  editor content explicitly, mirroring the rendered display.
- Keep pasted/typed URLs as plain text (linkPlugin disableAutoLink) so they
  don't become `[url](url)`, which duplicated both the URL and its preview
  card. Also dedupe extractUrls defensively so a repeated URL yields one card.

https://claude.ai/code/session_019DU6V5z6Nr4Vu7b1fBDnq4

js/desktop/src/lib/link-metadata.ts

@@ -10,5 +10,7 @@ export interface LinkMetadata {
 const URL_REGEX = /https?:\/\/[^\s<>"')\]]+/g;
 
 export function extractUrls(text: string): string[] {
-  return Array.from(text.matchAll(URL_REGEX), (m) => m[0]);
+  // Dedupe: a URL repeated in the text (e.g. a `[url](url)` markdown link)
+  // should only yield a single preview card.
+  return Array.from(new Set(Array.from(text.matchAll(URL_REGEX), (m) => m[0])));
 }