Add folder/notebook organization system

[Ditract]

Description

Implement a folder/notebook system to help users organize their notes hierarchically.

Features

• Create new folders/notebooks
• Rename existing folders
• Delete folders (with confirmation and handling of contained notes)
• Move notes between folders (drag & drop or selector)
• Display folder tree/list in sidebar
• Allow selecting destination folder when creating a new note (default: current folder or "Uncategorized")
• Support for nested folders

Technical considerations

• Handle notes when deleting folders (move to default or ask user)
• Nested folders will need recursive queries