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/components/UserAvatar.tsx

@@ -8,12 +8,14 @@ export interface UserAvatarProps {
   } | null;
   name?: string;
   avatarUrl?: string;
-  size?: 'xs' | 'sm' | 'md' | 'lg' | 'xl';
+  size?: 'xxs' | 'xs' | 'sm' | 'md' | 'lg' | 'xl';
   className?: string;
   alt?: string;
+  showTitle?: boolean; // native name tooltip on hover
 }
 
 const SIZE_MAP = {
+  xxs: 'w-4 h-4 text-[7px]',
   xs: 'w-5 h-5 text-[10px]',
   sm: 'w-7 h-7 text-xs',
   md: 'w-9 h-9 text-sm',
@@ -68,6 +70,7 @@ export const UserAvatar: React.FC<UserAvatarProps> = ({
   size = 'md',
   className = '',
   alt,
+  showTitle = true,
 }) => {
   const [imageError, setImageError] = useState(false);
 
@@ -94,7 +97,7 @@ export const UserAvatar: React.FC<UserAvatarProps> = ({
   return (
     <div
       className={`inline-flex items-center justify-center rounded-full aspect-square shrink-0 select-none overflow-hidden font-semibold transition-all ${sizeClass} ${className}`}
-      title={effectiveName || alt}
+      title={showTitle ? effectiveName || alt : undefined}
     >
       {hasImage ? (
         <img