🗑️ 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/helpers/ratio.js

@@ -1,20 +0,0 @@
-export const DEFAULT_ENDPOINT = '/api/ratio_config';
-
-/**
- * buildEndpointUrl: 拼接 baseUrl 与 endpoint，确保不会出现双斜杠或缺失斜杠问题。
- * 使用 URL 构造函数保证协议/域名安全；若 baseUrl 非标准 URL，则退回字符串拼接。
- * @param {string} baseUrl - 基础地址，例如 https://api.example.com
- * @param {string} endpoint - 接口路径，例如 /api/ratio_config
- * @returns {string}
- */
-export const buildEndpointUrl = (baseUrl, endpoint) => {
-  if (!baseUrl) return endpoint;
-  try {
-    return new URL(endpoint, baseUrl).toString();
-  } catch (_) {
-    // fallback 处理不规范的 baseUrl
-    const cleanedBase = baseUrl.endsWith('/') ? baseUrl.slice(0, -1) : baseUrl;
-    const cleanedEndpoint = endpoint.startsWith('/') ? endpoint.slice(1) : endpoint;
-    return `${cleanedBase}/${cleanedEndpoint}`;
-  }
-};