closes#91
3 mode config atuin history
```toml
[core]
# 0 = off, 1 = atuin history only, 2 = atuin history + default history
atuin-history = 0
```
Default is Mode 0 (turn off), who uses atuin history has to config it
manually
Mode 1 = atuin only (as you can see only 11 commands):
<img width="1285" height="466" alt="image"
src="https://github.com/user-attachments/assets/ca520ec7-fb45-4403-8087-a4951ba8dba0"
/>
Mode 2 = combine shell history + atuin history (prefer atuin):
<img width="1261" height="469" alt="image"
src="https://github.com/user-attachments/assets/1558ad3e-38f3-4d6e-bc8e-cfc00ea9cb4b"
/>
Mode 0 = turn off atuin
---
Custom atuin db path if anyone's using their own path (leave empty to
use default from atuin)
```toml
atuin-db-path = ""
```
---
I also handled the search menu overlay of Atuin, IRIS won't show in this
mode, but you have to change the keybind between 2 tool cuz they're
using same keybind Ctrl + R, but I think using IRIS make it unnecessary
to use Atuin search mode (82b180c9610f75d41008a54dfde474c7f2cce015)
<img width="70%" alt="image"
src="https://github.com/user-attachments/assets/e0c62050-7e6e-4e37-a1cc-2bfb63e1e984"
/>
<i><b>Before (Menu layout completely broken)</b></i>
<img width="50%" alt="image"
src="https://github.com/user-attachments/assets/1e6d8de9-cde8-473d-8a42-4748c480c812"
/>
<i ><b>After handling (not shows anymore)</b></i>
closes#112
Replace `os.UserConfigDir()` with explicit `~/.config` fallback to align
macOS, Linux, and other platforms on same config location
Respects `XDG_CONFIG_HOME` if set, otherwise uses `~/.config/iris/`
This project is starting to grow, and I've seen flaws in the initial
project structure design
I think I'll restructure the entire project directory for better
scalability and also cleaner
VERSE
