feat: drag onto file bookmark creates parent note + fix calendar layout

Drag-and-drop:
- Dropping a bookmark onto a file bookmark (middle zone) creates a new
  parent note group containing both items
- Dropping onto a group still moves into the group as before
- All file/group bookmarks now accept drops (3-zone behavior)

Calendar:
- Fixed broken table layout caused by display:flex on td elements
- Uses vertical-align:middle instead for cell content centering
- Calendar cells still have configurable height via CSS variable

styles.css

@@ -159,9 +159,7 @@
 	border-radius: 6px;
 	position: relative;
 	width: 100%;
-	display: flex;
-	align-items: center;
-	justify-content: center;
+	vertical-align: middle;
 }
 
 .waypoint-calendar .waypoint-day:hover {