Filters and Sort UI Improvement

### Describe your improvement idea or feature request

The current design uses “+” and “–” symbols to represent filtering and sorting order, which can be confusing for users and makes it difficult to understand how to achieve the desired ordering.

Also, requiring multiple clicks to cycle through the states (none → “+” → “–”) does not provide a user-friendly experience.


### How would you like to see it implemented?

Not exactly the same, but a good reference could be the sort feature used in this popular food delivery app, which I use frequently and which offers a relatively better user experience.


### Put here any screenshots or videos (optional)

_No response_

### How can we contact you (optional)

_No response_

### Would you like to work on this issue?

Yes

### By submitting this issue, I have confirmed that:

- [x] I have starred the repo ⭐ and watched 👀 it on GitHub and followed the contribution guidelines.

まず、Flutter モバイルアプリにある既存のフィルターおよび並べ替えコントロールと、この issue での議論を確認します。「+」と「–」の状態に代わる具体的な案について合意し、そのうえで、並べ替えとフィルタリングが分かりやすく、必要なクリック数が少なくなっていることを確認します。

索引モデルが issue の本文から書いたものです。