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/OutlinerTree.tsx

@@ -67,6 +67,12 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
   const [draggedIndex, setDraggedIndex] = useState<number | null>(null);
   const [dragOverIndex, setDragOverIndex] = useState<number | null>(null);
 
+  // Leaving a block (click into empty space) must drop its focus highlight; a focus move to
+  // another block has already set the new id, so only clear if it's still this one
+  const handleBlurBlock = useCallback((blockId: string) => {
+    setFocusedBlockId((cur) => (cur === blockId ? null : cur));
+  }, []);
+
   const textareaMapRef = useRef<Map<string, HTMLTextAreaElement>>(new Map());
   const pendingFocusRef = useRef<{ blockId: string; cursor: 'start' | 'end' | 'select' | number } | null>(null);
 
@@ -785,6 +791,7 @@ export const OutlinerTree = forwardRef<OutlinerTreeHandle, OutlinerTreeProps>(({
             onIndentBlock={handleIndentBlock}
             onNavigateFocus={handleNavigateFocus}
             onFocusBlock={setFocusedBlockId}
+            onBlurBlock={handleBlurBlock}
             onOpenTask={onOpenTask}
             onTagClick={onTagClick}
             onWikiLinkClick={onWikiLinkClick}