Feature: Folder Support

[gab-dev-7]

Currently, Kiroku works on a flat file structure. Adding support for subdirectories would allow for much better organization (e.g., work/, personal/, projects/).

Requirements:

• Update Note struct to handle relative paths.
• Implement a 'Folder View' or tree structure in the file list.
• Allow creating notes in subdirectories (e.g., work/project_a.md).
• Update search to include folder names.

[gab-dev-7]

#7 done.