Summary

• Default the friend-list widget display mode to "Grouped by Folder" so folders appear without manual configuration (matching the block theme sidebar behavior)
• Fix array_merge order so per-folder accordion widget IDs aren't overwritten by sidebar args (each folder's open/close state now tracked independently)
• Show the folder name as headline when filtering by a specific folder, in both the classic widget and the friends-list block

Test plan

• Add the Friend List widget to the sidebar, verify it defaults to "Grouped by Folder" mode
• Create folders and assign subscriptions, verify they appear grouped in the widget
• In widget settings, choose "Subscriptions" display + pick a specific folder, verify the folder name shows as the title
• In the block editor, add a Friends List block with a folder attribute, verify the folder name heading appears

Test on WordPress Playground