Add support for customizable terminal shell configuration

Add support for customizable terminal shell configuration (e.g., zsh, git bash, bash)

**Description:**

Currently, the terminal only supports the default system shell. Users should be able to configure their preferred shell environment (e.g., zsh, git bash, bash, PowerShell, etc.) in the terminal settings.

---
| Field | Value |
| --- | --- |
| App version | 1.1.10 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Day 4 (Week 1) |

Start by locating the terminal settings UI and the code that launches the default system shell. Trace how shell selection is stored and passed to the terminal, including the listed zsh, Git Bash, bash, and PowerShell examples. Done means users can choose a preferred shell in terminal settings and new terminal sessions use that choice.

索引モデルが issue の本文から書いたものです。