### What problem does this PR solve?
This PR closing feature request #11286.
It implements ability to choose the background theme of the _Full screen
chat_ which is Embed into webpage.
Looks like that:
<img width="501" height="349" alt="image"
src="https://github.com/user-attachments/assets/e5fdfb14-9ed9-43bb-a40d-4b580985b9d4"
/>
It works similar to `Locale`, using url parameter to set the theme.
if the parameter is invalid then is using the default theme.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
---------
Co-authored-by: Your Name <you@example.com>
### What problem does this PR solve?
Fix: Switch the default theme from light mode to dark mode and improve
some styles #9869
-Update UI component styles such as input boxes, tables, and prompt
boxes
-Optimize login page layout and style details
-Revise some of the wording, such as uniformly changing "data flow" to
"pipeline"
-Adjust the parser to support the markdown type
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Feat: Automatically save agent canvas content
Feat: Replace the link of the old version of the agent module #3221
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Feat: Bind event to the theme Switch #3221
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Feat: Set the color of the canvas's control button #3851
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
- [x] New Feature (non-breaking change which adds functionality)
---------
Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>
### What problem does this PR solve?
feat: Add next login page with shadcn/ui #3221
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
