feat: drag-and-drop into groups/parent notes + fix ⋯ button styling

Drag-and-drop:
- 3-zone drop indicator for groups and parent notes (files with children):
  top 25% = drop above, middle 50% = drop into, bottom 25% = drop below
- 2-zone for regular files/separators/spacers (unchanged)
- 'Drop into' shows background highlight instead of border line
- Circular reference guard: can't drop a group into itself or descendants

⋯ button:
- No visible button chrome when not hovered (no border, no background)
- Subtle opacity (0.6) at rest, full opacity on hover
- Hover shows text color change only, no background highlight

styles.css

@@ -44,11 +44,14 @@
 	align-items: center;
 	justify-content: center;
 	transition: color 80ms, background 80ms;
+	box-shadow: none;
+	opacity: 0.6;
 }
 
 .waypoint-header-more:hover {
-	color: var(--text-accent);
-	background-color: var(--background-modifier-active-hover);
+	color: var(--text-normal);
+	background: none;
+	opacity: 1;
 }
 
 /* ── Calendar ── */
@@ -287,6 +290,11 @@
 	border-bottom: 3px solid var(--text-accent) !important;
 }
 
+.waypoint-favorites .waypoint-bm-drop-into {
+	background-color: var(--background-modifier-active-hover) !important;
+	border-radius: 4px;
+}
+
 /* ── Separator & Spacer ── */
 
 .waypoint-favorites .waypoint-bookmark-item.waypoint-bookmark-separator {