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/AudioBlock.tsx

@@ -87,7 +87,7 @@ export const AudioBlock: React.FC<AudioBlockProps> = ({ content, canEdit, onCapt
   return (
     <div className="my-1 space-y-1">
       {failed ? (
-        <div className="flex items-center gap-2 h-12 px-3 rounded-card border border-border bg-panel text-xs text-muted">
+        <div className="flex items-center gap-2 h-12 px-4 rounded-full border border-border bg-panel text-xs text-muted">
           <FileAudio className="w-4 h-4" /> Aufnahme nicht verfügbar (offline oder gelöscht)
         </div>
       ) : src ? (
@@ -99,7 +99,7 @@ export const AudioBlock: React.FC<AudioBlockProps> = ({ content, canEdit, onCapt
               onClick={() => void transcribe()}
               disabled={transcribing}
               title="In Text umwandeln (eigene Spracherkennung)"
-              className="btn-secondary h-11 sm:h-8 px-2.5 text-xs flex items-center gap-1.5 shrink-0 disabled:opacity-40"
+              className="btn-secondary !rounded-full h-11 sm:h-8 px-3 text-xs flex items-center gap-1.5 shrink-0 disabled:opacity-40"
             >
               {transcribing ? <Loader2 className="w-3.5 h-3.5 animate-spin" /> : <Wand2 className="w-3.5 h-3.5" />}
               <span className="hidden sm:inline">{transcribing ? 'Wird erkannt…' : 'Transkribieren'}</span>
@@ -107,7 +107,7 @@ export const AudioBlock: React.FC<AudioBlockProps> = ({ content, canEdit, onCapt
           )}
         </div>
       ) : (
-        <div className="flex items-center h-12 px-3 rounded-card border border-border bg-panel w-64">
+        <div className="flex items-center h-12 px-4 rounded-full border border-border bg-panel w-64">
           <Loader2 className="w-4 h-4 animate-spin text-muted" />
         </div>
       )}