Feat: Add frontend support for third-party login integration (#7553)

### 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


![image](https://github.com/user-attachments/assets/190ad3a5-3718-409a-ad0e-01e7aca39069)

```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

web/src/services/user-service.ts

@@ -123,6 +123,10 @@ const methods = {
 
 const userService = registerServer<keyof typeof methods>(methods, request);
 
+export const getLoginChannels = () => request.get(api.login_channels);
+export const loginWithChannel = (channel: string) =>
+  (window.location.href = api.login_channel(channel));
+
 export const listTenantUser = (tenantId: string) =>
   request.get(api.listTenantUser(tenantId));