refactor: simplify bookmark menus — visible ⋯ button, flat context menus

Header:
- Replace hidden right-click with visible ⋯ button (more-horizontal icon)
- Menu: Add current file / Add as parent note / New group / separator / Add spacer

File bookmark right-click (5 items, flat):
  Open in new tab / Rename / Change icon / Remove

Group/parent note right-click (7 items max, flat):
  Open in new tab (only if linked) / Rename / Change icon / Expand|Collapse /
  Add child bookmark / Add child note / New sub-group / Remove

Separator/Spacer right-click (1 item):
  Remove

Removed dead code: insertAdjacent, collectGroups, collectDescendantIds,
moveBookmarkToGroup — all moving is now drag-and-drop only

todo.md

@@ -3,6 +3,8 @@
   - [ ] in the bookmarks, i want to have notes that are parents to other notes 
   - [ ] by default, the bookmark should have no icon
   - [ ] the way to search for icons should be improved. on the lucide icon website, the search works by more than only the icon’s title. i want to be able to load all the icon (not by default though)
+  - [ ] add a right click menu on the recent files to make them bookmarks
+  - [ ] improve the drag and drop to have identations and groups
 - [ ] calendar
   - [ ] middle click on a date/week/etc. should open it in a new tab
   - [ ] i want to improve the look of the calendar. more space, a bit more height