[FEATURE]: Add a Back to Top button

### Feature and its Use Cases

## Feature Description

Add a "Back to Top" button that appears when users scroll down the page.

The button should allow users to quickly return to the top of the page by clicking it. The page should scroll back to the top smoothly.

## Use Cases

- Users can quickly return to the top of long pages without manually scrolling.
- It improves navigation and overall user experience.
- It is especially useful on pages with a large amount of content.
- The button can appear after the user has scrolled down a certain distance and remain hidden when the user is already near the top.

### Additional Context

A reusable Back to Top component could be implemented so that it can be used across pages where long scrolling is required.

The button should follow the existing UI design and remain unobtrusive. It should also work properly on both desktop and mobile screens.

The scroll behavior should be smooth when the user clicks the button.

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

Start by locating the page layout and existing UI components or styles used across OrgExplorer, then check how long pages behave on desktop and mobile. Add a reusable Back to Top component that appears after scrolling, stays hidden near the top, follows the existing design, and returns users smoothly to the top when clicked.

索引モデルが issue の本文から書いたものです。