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/OutlinerBlock.tsx

@@ -62,6 +62,7 @@ export interface OutlinerBlockProps {
   onIndentBlock: (blockId: string, delta: 1 | -1) => void;
   onNavigateFocus: (blockId: string, direction: 'up' | 'down') => void;
   onFocusBlock?: (blockId: string) => void;
+  onBlurBlock?: (blockId: string) => void;
   onOpenTask?: (taskId: string) => void;
   onTagClick?: (tag: string) => void;
   onWikiLinkClick?: (pageTitle: string) => void;
@@ -109,6 +110,7 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
   onIndentBlock,
   onNavigateFocus,
   onFocusBlock,
+  onBlurBlock,
   onOpenTask,
   onTagClick,
   onWikiLinkClick,
@@ -409,6 +411,7 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
   const handleBlur = () => {
     isFocusedRef.current = false;
     setIsLocallyFocused(false);
+    onBlurBlock?.(block.id);
     if (isPastingRef.current) {
       isPastingRef.current = false;
       return;
@@ -668,16 +671,6 @@ export const OutlinerBlock: React.FC<OutlinerBlockProps> = ({
         }
       }}
     >
-      {/* Indentation Guide Line */}
-      {visualLevel > 0 &&
-        Array.from({ length: visualLevel }).map((_, lvlIdx) => (
-          <div
-            key={lvlIdx}
-            className="pointer-events-none absolute top-0 bottom-0 w-px border-l border-border"
-            style={{ left: `${lvlIdx * 22 + 12}px` }}
-          />
-        ))}
-
       {/* Bullet Controls: Collapse Toggle & Bullet (Drag-Handle & Zoom) / Checkbox */}
       <div className="flex items-center gap-0.5 pt-1 select-none shrink-0">
         {hasChildren ? (