feat(provider): add required field validation with toast notifications

- Add validation for provider name (required for all providers)
- Add validation for API endpoint and API Key (required for non-official providers)
- Use toast notifications instead of inline form errors for better visibility
- Move name validation from zod schema to handleSubmit for consistent UX
- Add i18n keys: endpointRequired, apiKeyRequired

src/lib/schemas/provider.ts

@@ -36,7 +36,7 @@ function parseJsonError(error: unknown): string {
 }
 
 export const providerSchema = z.object({
-  name: z.string().min(1, "请填写供应商名称"),
+  name: z.string(), // 必填校验移至 handleSubmit 中用 toast 提示
   websiteUrl: z.string().url("请输入有效的网址").optional().or(z.literal("")),
   notes: z.string().optional(),
   settingsConfig: z