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/features/compose/markdown-editor.css

@@ -1,7 +1,12 @@
-/* Blend MDXEditor into the compose glass card.
-   We lean on MDXEditor's bundled `dark-theme` for typography (heading sizes,
-   list markers, code styling) and only override the chrome: transparent
-   background, no border, and padding/colors tuned to the surrounding card. */
+/* Blend MDXEditor into the compose glass card, and restore prose typography.
+ *
+ * MDXEditor renders real <h1>/<ul>/<blockquote> elements and leans on the
+ * browser's default styles for their look — it ships no heading/list
+ * typography of its own. Tailwind's Preflight resets those defaults
+ * (headings -> `font-size: inherit`, lists -> `list-style: none`), which would
+ * otherwise make a heading or bullet look identical to body text. So we style
+ * the content elements explicitly here, mirroring the rendered display in
+ * text-particle-view.tsx so "write" and "read" match. */
 
 .llink-mdxeditor {
   --baseBg: transparent;
@@ -24,10 +29,106 @@
   line-height: 1.7;
 }
 
-.llink-mdx-content :where(h1, h2, h3, h4, h5, h6) {
+/* Headings */
+.llink-mdx-content h1,
+.llink-mdx-content h2,
+.llink-mdx-content h3,
+.llink-mdx-content h4,
+.llink-mdx-content h5,
+.llink-mdx-content h6 {
   color: #fff;
+  line-height: 1.25;
+}
+.llink-mdx-content h1 {
+  font-size: 1.875rem;
+  font-weight: 700;
+  margin-bottom: 0.75rem;
+}
+.llink-mdx-content h2 {
+  font-size: 1.5rem;
+  font-weight: 600;
+  margin-bottom: 0.5rem;
+}
+.llink-mdx-content h3 {
+  font-size: 1.25rem;
+  font-weight: 600;
+  margin-bottom: 0.5rem;
+}
+.llink-mdx-content h4 {
+  font-size: 1.125rem;
+  font-weight: 500;
+  margin-bottom: 0.25rem;
+}
+.llink-mdx-content h5 {
+  font-size: 1rem;
+  font-weight: 500;
+  margin-bottom: 0.25rem;
+}
+.llink-mdx-content h6 {
+  font-size: 0.875rem;
+  font-weight: 500;
+  margin-bottom: 0.25rem;
 }
 
+/* Block spacing */
+.llink-mdx-content p {
+  margin-bottom: 0.75rem;
+  line-height: 1.625;
+}
+.llink-mdx-content > :last-child {
+  margin-bottom: 0;
+}
+
+/* Inline emphasis */
+.llink-mdx-content strong {
+  font-weight: 600;
+  color: #fff;
+}
+.llink-mdx-content em {
+  font-style: italic;
+}
 .llink-mdx-content a {
   color: #60a5fa;
+  text-decoration: underline;
+}
+
+/* Lists — Preflight strips the marker and indentation, so restore both */
+.llink-mdx-content ul,
+.llink-mdx-content ol {
+  margin-bottom: 0.75rem;
+  padding-left: 1.25rem;
+}
+.llink-mdx-content ul {
+  list-style: disc;
+}
+.llink-mdx-content ol {
+  list-style: decimal;
+}
+.llink-mdx-content li {
+  margin-bottom: 0.25rem;
+  line-height: 1.625;
+}
+
+/* Blockquote */
+.llink-mdx-content blockquote {
+  margin-bottom: 0.75rem;
+  padding-left: 1rem;
+  border-left: 2px solid rgb(255 255 255 / 0.3);
+  font-style: italic;
+  color: rgb(255 255 255 / 0.7);
+}
+
+/* Inline code */
+.llink-mdx-content code {
+  border-radius: 0.25rem;
+  background: rgb(255 255 255 / 0.1);
+  padding: 0.1rem 0.375rem;
+  font-size: 0.85em;
+}
+
+/* Horizontal rule */
+.llink-mdx-content hr {
+  margin: 1rem 0;
+  border: none;
+  border-top: 1px solid rgb(255 255 255 / 0.1);
 }