✨ Git branches selector quality of life improvements

### Description

I think two very important features are missing from the existing Git branches selector/dropdown in order to be usable on large repositories.

1. A search/filter bar
2. Nested branches, for example `feat/my-branch` and `feat/my-other-branch` should both live under a common `feat` parent folder

### Alternatives Considered

_No response_

### Additional Context

_No response_

### Screenshots

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從現有的 Git 分支選擇器/下拉選單著手，追蹤分支的載入與呈現方式。完成的定義是：選擇器支援篩選，並將 feat/my-branch 之類的斜線分隔分支名稱顯示在共用的 feat 父層下；issue 中未指定特定檔案或測試。

由索引模型根據 Issue 內容生成。