### What problem does this PR solve?
Add frontend support for third-party login integration:
- Used `getLoginChannels` API to fetch available login channels from the
server
- Used `loginWithChannel` function to initiate login based on the
selected channel
- Refactored `useLoginWithGithub` hook to `useOAuthCallback` for
generalized OAuth callback handling
- Updated the login page to dynamically render third-party login buttons
based on the fetched channel list
- Styled third-party login buttons to improve user experience
- Removed unused code snippets
> This PR removes the previously hardcoded GitHub login button. Since
the functionality only worked when `location.host` was equal to
`demo.ragflow.io`, and the authentication logic is now based on
`login.ragflow.io`, this change does not affect the existing logic and
is considered a non-breaking change
---
#### Frontend Screenshot && Backend Configuration
```yaml
# docker/service_conf.yaml.template
# ...
oauth:
github:
icon: github
display_name: "Github"
# ...
custom_channel:
display_name: "OIDC"
# ...
custom_channel_2:
display_name: "OAuth2"
# ...
```
---
- Related pull requests:
- #7379
- #7521
- Related issues:
- #3495
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
- [x] Refactoring
- [x] Performance Improvement
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