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

styles.css

@@ -27,6 +27,28 @@
 	margin-bottom: 4px;
 	padding-bottom: 4px;
 	border-bottom: 1px solid var(--background-modifier-border);
+	display: flex;
+	align-items: center;
+	gap: 6px;
+}
+
+.waypoint-header-more {
+	margin-left: auto;
+	background: none;
+	border: none;
+	cursor: var(--cursor);
+	color: var(--text-faint);
+	padding: 2px 4px;
+	border-radius: 4px;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	transition: color 80ms, background 80ms;
+}
+
+.waypoint-header-more:hover {
+	color: var(--text-accent);
+	background-color: var(--background-modifier-active-hover);
 }
 
 /* ── Calendar ── */