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/DictationBlock.tsx

@@ -85,7 +85,7 @@ export const DictationBlock: React.FC<DictationBlockProps> = ({ onDone, onCancel
       }}
       disabled={transcribing || !!error}
       aria-label="Diktat beenden"
-      className="relative w-full min-h-[56px] sm:min-h-[44px] my-1 px-3 py-2 rounded-card border border-accent bg-accent-dim overflow-hidden flex flex-col gap-1.5 text-left cursor-pointer disabled:cursor-default focus:outline-none"
+      className="relative w-full min-h-[56px] sm:min-h-[44px] my-1 px-4 py-2 rounded-3xl border border-accent bg-accent-dim overflow-hidden flex flex-col gap-1.5 text-left cursor-pointer disabled:cursor-default focus:outline-none"
     >
       {(interim || chunkBusy) && !error && (
         <div className="w-full text-sm text-main whitespace-pre-wrap break-words leading-relaxed pointer-events-none">