feat(workspace): borderless code blocks, round code/audio/dictation blocks

Code blocks lose border and header bar: language at the top left, a round
copy button at the bottom right on hover; converting back to text moves to
the bullet's action bubble (also for numbered items). Code, audio and
dictation blocks use fixed round corners whatever radius the theme sets.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/OutlinerBlock.tsx

@@ -1613,8 +1613,8 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
             : 'opacity-0 group-hover:opacity-100 focus-within:opacity-100 pointer-events-none group-hover:pointer-events-auto focus-within:pointer-events-auto'
         }`}
       >
-        {/* Convert back to bullet for headings, callouts, divider */}
-        {canEdit && ['callout', 'h1', 'h2', 'h3', 'divider'].includes(block.type) && (
+        {/* Convert back to bullet for headings, callouts, dividers, code and numbered items */}
+        {canEdit && ['callout', 'h1', 'h2', 'h3', 'divider', 'code', 'numbered'].includes(block.type) && (
           <button
             type="button"
             onClick={() => {