Improve UI consistency of "Set Default" and "Change Directory" buttons in Settings

## Description

In the **Settings → Storage and Data** section, the buttons **"Set To Default"** and **"Change Directory"** appear as default Flutter buttons.

They look visually inconsistent with the rest of the settings UI, which uses rounded cards and toggle-style controls.

## Current Behavior

The buttons appear large and flat, making them look different from the rest of the interface.

## Proposed Improvement

Improve the styling of these buttons so they better match the existing design language.

Possible improvements:

* Use rounded or pill-style buttons
* Improve spacing and layout
* Make "Change Directory" the primary action

## Screenshot


If this change is acceptable, I would be happy to work on implementing it.

まず Settings → Storage and Data セクションを見つけ、既存の角丸カードとトグルスタイルのコントロールがどのように構築されているかを確認します。次に、「Set To Default」と「Change Directory」ボタンのスタイルをこのデザイン言語に合わせて更新し、Settings UI でのスペーシング、外観、主要アクションの階層を確認します。

索引モデルが issue の本文から書いたものです。