refactor: improve endpoint management type safety and error handling

- Unify EndpointCandidate type definition in types.ts
- Remove all 'as any' type assertions in useSpeedTestEndpoints
- Add cleanup function to prevent race conditions in async operations
- Fix stale error messages persisting after successful deletion
- Improve error handling for endpoint deletion (distinguish not-found vs network errors)
- Extract timeout magic numbers to ENDPOINT_TIMEOUT_SECS constant
- Clarify URL validation to explicitly allow only http/https
- Fix ambiguous payload.meta assignment logic in ProviderForm
- Add i18n for new error messages (removeFailed, updateLastUsedFailed)

src/types.ts

@@ -30,6 +30,13 @@ export interface CustomEndpoint {
   lastUsed?: number;
 }
 
+// 端点候选项（用于端点测速弹窗）
+export interface EndpointCandidate {
+  id?: string;
+  url: string;
+  isCustom?: boolean;
+}
+
 // 用量查询脚本配置
 export interface UsageScript {
   enabled: boolean; // 是否启用用量查询