fix: Nested background threads leaking

[dmtrKovalenko]

fixes #49

[Copilot]

Pull Request Overview

This PR fixes a critical issue where background threads in the file picker were leaking when the application was shut down or reinitialized. The fix implements proper cleanup mechanisms using a debouncer-based approach instead of the previous shutdown signal pattern.

• Replaces the shutdown signal approach with proper resource management using a debouncer wrapper
• Adds explicit cleanup functionality for stopping background monitors and cleaning up file pickers
• Implements automatic cleanup on Neovim exit via autocmd

Reviewed Changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lua/fff/rust/file_picker.rs	Refactors background thread management to use debouncer-based cleanup instead of shutdown signals
lua/fff/rust/lib.rs	Adds cleanup functions and updates error handling for file picker operations
lua/fff/rust/error.rs	Simplifies error types and adds proper error logging
lua/fff/rust/tracing.rs	Simplifies error handling for directory creation
lua/fff/main.lua	Adds VimLeavePre autocmd for automatic cleanup and refactors function naming
lua/fff/fuzzy.lua	Exposes new cleanup_file_picker function

Comments suppressed due to low confidence (1)

lua/fff/rust/file_picker.rs:167

• Explicit drop(picker) is redundant since take() already transfers ownership and the value will be dropped automatically at the end of the scope.

    fn from(file: &FileItem) -> Self {