feat(notes): presence as avatar stack, drop guide lines, fix stuck bullet focus

- Leaving a block now clears its focus, so the bullet no longer stays highlighted
- Remove the vertical indentation guide lines
- Who has the note open: up to 3 overlapping avatars, "+n", full list on hover
- Shared AvatarStack component; UserAvatar gets xxs size and optional title

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

frontend/src/features/workspace/PresenceAvatars.tsx

@@ -0,0 +1,41 @@
+import React from 'react';
+import { UserAvatar } from '../../components/UserAvatar';
+import { AvatarStack } from '../../components/AvatarStack';
+import { Presence } from '../../lib/docSync/transport';
+
+const MAX_VISIBLE = 3;
+
+interface PresenceAvatarsProps {
+  people: Presence[];
+  avatars: Map<string, string>; // userId -> avatarUrl
+}
+
+// Who else has the note open: the shared AvatarStack plus the full list on hover / keyboard focus
+export const PresenceAvatars: React.FC<PresenceAvatarsProps> = ({ people, avatars }) => {
+  if (people.length === 0) return null;
+  return (
+    <div
+      className="group/presence relative flex items-center outline-none"
+      tabIndex={0}
+      aria-label={`Gerade online: ${people.map((p) => p.name).join(', ')}`}
+    >
+      <AvatarStack
+        people={people.map((p) => ({ id: p.userId, name: p.name, avatarUrl: avatars.get(p.userId) }))}
+        max={MAX_VISIBLE}
+      />
+
+      <div className="invisible opacity-0 group-hover/presence:visible group-hover/presence:opacity-100 group-focus/presence:visible group-focus/presence:opacity-100 transition-opacity absolute right-0 top-full mt-1.5 z-30 min-w-[11rem] max-w-[16rem] p-1.5 rounded-card bg-panel border border-border shadow-lg">
+        <div className="px-1.5 pb-1 text-[10px] uppercase tracking-wide text-faint">Gerade online</div>
+        {people.map((p) => (
+          <div key={p.userId} className="flex items-center gap-2 px-1.5 py-1 text-xs text-main">
+            <span className="relative">
+              <UserAvatar name={p.name} avatarUrl={avatars.get(p.userId) || ''} size="xs" />
+              <span className="absolute -bottom-0.5 -right-0.5 w-2 h-2 rounded-full bg-status-ok ring-2 ring-[var(--bg-panel)]" />
+            </span>
+            <span className="truncate">{p.name}</span>
+          </div>
+        ))}
+      </div>
+    </div>
+  );
+};