feat: parent notes (file bookmarks with children) + no default icon

- File bookmarks can now have children, rendered with a chevron
- Group bookmarks can have an associated filePath; clicking opens the file,
  clicking chevron toggles collapse
- 'Add as parent note' option in header context menu creates a group
  with the current file linked
- 'Add child note' option in group context menu creates a sub-group
  with the current file linked (parent note pattern)
- Default icon changed from 'file'/'folder' to '' (no icon)
- Chevron click always toggles collapse regardless of type

src/main.ts

@@ -254,7 +254,7 @@ export default class WaypointPlugin extends Plugin {
 
 	// ── Bookmarks ──
 
-	addBookmark(filePath: string, label: string, type: 'file' | 'group' | 'separator' | 'spacer' = 'file', icon = 'file'): BookmarkItem {
+	addBookmark(filePath: string, label: string, type: 'file' | 'group' | 'separator' | 'spacer' = 'file', icon = ''): BookmarkItem {
 		const id = `bm-${Date.now()}-${Math.random().toString(36).slice(2, 6)}`;
 		const item: BookmarkItem = {
 			id,