🗑️ chore(custom channel): Remove custom channel support from upstream ratio sync

Remove all custom channel functionality from the upstream ratio sync feature to simplify the codebase and focus on database-stored channels only.

Changes:
- Remove custom channel UI components and related state management
- Remove custom channel testing and validation logic
- Simplify ChannelSelectorModal by removing custom channel input fields
- Update API payload to only include channel_ids, removing custom_channels
- Remove custom channel processing logic from backend controller
- Update import path for DEFAULT_ENDPOINT constant

Files modified:
- web/src/pages/Setting/Ratio/UpstreamRatioSync.js
- web/src/components/settings/ChannelSelectorModal.js
- controller/ratio_sync.go

This change streamlines the ratio synchronization workflow by focusing solely on pre-configured database channels, reducing complexity and potential maintenance overhead.

web/src/constants/common.constant.js

@@ -1 +1,3 @@
 export const ITEMS_PER_PAGE = 10; // this value must keep same as the one defined in backend!
+
+export const DEFAULT_ENDPOINT = '/api/ratio_config';