refactor(url): remove automatic trailing slash stripping from base URL inputs

- Remove `.replace(/\/+$/, "")` from all base URL handlers in useBaseUrlState.ts
- Remove trailing slash stripping in useCodexConfigState.ts
- Remove trailing slash normalization in providerConfigUtils.ts setCodexBaseUrl()
- Update i18n hints (en/ja/zh) to instruct users to avoid trailing slashes

This gives users explicit control over URL format rather than silently modifying input.

src/components/providers/forms/hooks/useCodexConfigState.ts

@@ -162,7 +162,7 @@ export function useCodexConfigState({ initialData }: UseCodexConfigStateProps) {
   // 处理 Codex Base URL 变化
   const handleCodexBaseUrlChange = useCallback(
     (url: string) => {
-      const sanitized = url.trim().replace(/\/+$/, "");
+      const sanitized = url.trim();
       setCodexBaseUrl(sanitized);
 
       if (!sanitized) {