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/components/providers/forms/ProviderForm.tsx

@@ -456,6 +456,7 @@ export function ProviderForm({
             categoryKeys={categoryKeys}
             presetCategoryLabels={presetCategoryLabels}
             onPresetChange={handlePresetChange}
+            category={category}
           />
         )}