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/AudioRecorderPicker.tsx

@@ -108,7 +108,7 @@ export const AudioRecorderPicker: React.FC<AudioRecorderPickerProps> = ({ docId,
           cancel();
         }
       }}
-      className="my-1 inline-flex max-w-full flex-col gap-2 rounded-card border border-border bg-panel-elevated px-3 py-2 focus:outline-none"
+      className="my-1 inline-flex max-w-full flex-col gap-2 rounded-3xl border border-border bg-panel-elevated px-4 py-2 focus:outline-none"
     >
       <div className="flex items-center gap-3">
         {state === 'recording' && (
@@ -162,7 +162,7 @@ export const AudioRecorderPicker: React.FC<AudioRecorderPickerProps> = ({ docId,
           onClick={cancel}
           disabled={state === 'uploading'}
           title="Abbrechen (Esc)"
-          className="w-11 h-11 sm:w-7 sm:h-7 flex items-center justify-center rounded-btn text-muted hover:text-main hover:bg-panel shrink-0 disabled:opacity-40"
+          className="w-11 h-11 sm:w-7 sm:h-7 flex items-center justify-center rounded-full text-muted hover:text-main hover:bg-panel shrink-0 disabled:opacity-40"
         >
           <X className="w-4 h-4" />
         </button>