Complete GFM support: tables, task lists, strikethrough

Editor: add tablePlugin so markdown tables round-trip and render. Task
lists (`- [ ]`), strikethrough, code (inline + fenced), headings, lists,
quotes, and links already work via listsPlugin/core/markdownShortcut. CSS:
suppress the disc bullet on task-list items (they draw their own checkbox)
and style tables.

Display: render task-list checkboxes (no stray bullet), GFM tables, and
strikethrough so the read view matches what the editor produces.

https://claude.ai/code/session_019DU6V5z6Nr4Vu7b1fBDnq4

js/desktop/src/features/compose/markdown-editor.css

@@ -108,6 +108,10 @@
   margin-bottom: 0.25rem;
   line-height: 1.625;
 }
+/* Task-list items draw their own checkbox — no disc bullet. */
+.llink-mdx-content li[class*="_listItem"] {
+  list-style: none;
+}
 
 /* Blockquote */
 .llink-mdx-content blockquote {
@@ -132,3 +136,20 @@
   border: none;
   border-top: 1px solid rgb(255 255 255 / 0.1);
 }
+
+/* Tables */
+.llink-mdx-content table {
+  margin-bottom: 0.75rem;
+  border-collapse: collapse;
+  width: auto;
+}
+.llink-mdx-content th,
+.llink-mdx-content td {
+  border: 1px solid rgb(255 255 255 / 0.2);
+  padding: 0.375rem 0.625rem;
+  text-align: left;
+}
+.llink-mdx-content th {
+  background: rgb(255 255 255 / 0.08);
+  font-weight: 600;
+}