feat: add category-specific hints to provider form

- Add dynamic hint text based on provider category (official, opensource, aggregator, third-party, custom)
- Display hints in ProviderPresetSelector below preset buttons
- Rename "Chinese official" to "Opensource official" for better clarity
- Add i18n keys for all category hints in both zh and en locales
- Remove redundant hint boxes from ClaudeFormFields and CodexFormFields

This improves user guidance by showing contextual hints that explain
what fields are required for each provider category.

src/types.ts

@@ -1,6 +1,6 @@
 export type ProviderCategory =
   | "official" // 官方
-  | "cn_official" // 国产官方
+  | "cn_official" // 开源官方（原"国产官方"）
   | "aggregator" // 聚合网站
   | "third_party" // 第三方供应商
   | "custom"; // 自定义