feat: added a delete feature to the program

[TNhelly]

Summary

Adds delete functionality

Changes

• Added Action::Delete variant
• Added delete_current_card() method to App
• Added delete_card() to store_fs.rs (removes .md file + updates order.txt)
• Added delete_card() to Provider trait and LocalProvider impl
• Added 'd' key binding in main.rs
• Updated help text

Testing

• Card disappears from UI when 'd' is pressed
• .md file is removed from disk
• Card ID is removed from order.txt
• Board reloads without errors after deletion
• Error handling for missing cards/providers