fix: RTL issues in new conversation form (#12163)

# Pull Request Template

## Description

This PR fixes RTL alignment issues in the new conversation form, removes
the unused
[`form-checkbox`](https://github.com/chatwoot/chatwoot/pull/12151#discussion_r2266333315)
class name and drops the `app-rtl--wrapper` class, which was previously
used for RTL detection in `rtl.scss` (removed earlier)

Fixes https://linear.app/chatwoot/issue/CW-5410/rtl-issues

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

### Screenshots

<img width="868" height="474" alt="image"
src="https://github.com/user-attachments/assets/45995652-2895-49d5-a651-179090c949ec"
/>
<img width="868" height="656" alt="image"
src="https://github.com/user-attachments/assets/a1cb4415-3fd4-4c9a-bc46-5e07e437d757"
/>
<img width="868" height="656" alt="image"
src="https://github.com/user-attachments/assets/77c8981f-364e-4bf0-bea8-a4c42a76d065"
/>



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/components-next/sidebar/SidebarGroup.vue

@@ -218,8 +218,8 @@ onMounted(async () => {
   left: 0;
 }
 
-.app-rtl--wrapper .sidebar-group-children > .child-item:last-child::after,
-.app-rtl--wrapper
+#app[dir='rtl'] .sidebar-group-children > .child-item:last-child::after,
+#app[dir='rtl']
   .sidebar-group-children
   > *:last-child
   > *:last-child