refactor: split ProviderForm into smaller focused components

- Created ProviderPresetSelector component (80 lines)
- Created BasicFormFields component (60 lines)
- Created ClaudeFormFields component (272 lines)
- Created CodexFormFields component (131 lines)
- Reduced ProviderForm from 866 to 544 lines (37% reduction)

Each component now has a clear single responsibility:
- ProviderPresetSelector: Handles preset selection UI
- BasicFormFields: Name and website URL inputs
- ClaudeFormFields: All Claude-specific form fields
- CodexFormFields: All Codex-specific form fields
- ProviderForm: Orchestrates hooks and component composition

Benefits:
- Better code organization and maintainability
- Easier to test individual components
- Clearer separation of concerns
- More reusable components

src/components/providers/forms/BasicFormFields.tsx

@@ -0,0 +1,60 @@
+import { useTranslation } from "react-i18next";
+import {
+  FormControl,
+  FormField,
+  FormItem,
+  FormLabel,
+  FormMessage,
+} from "@/components/ui/form";
+import { Input } from "@/components/ui/input";
+import type { UseFormReturn } from "react-hook-form";
+import type { ProviderFormData } from "@/lib/schemas/provider";
+
+interface BasicFormFieldsProps {
+  form: UseFormReturn<ProviderFormData>;
+}
+
+export function BasicFormFields({ form }: BasicFormFieldsProps) {
+  const { t } = useTranslation();
+
+  return (
+    <>
+      <FormField
+        control={form.control}
+        name="name"
+        render={({ field }) => (
+          <FormItem>
+            <FormLabel>
+              {t("provider.name", { defaultValue: "供应商名称" })}
+            </FormLabel>
+            <FormControl>
+              <Input
+                {...field}
+                placeholder={t("provider.namePlaceholder", {
+                  defaultValue: "例如：Claude 官方",
+                })}
+              />
+            </FormControl>
+            <FormMessage />
+          </FormItem>
+        )}
+      />
+
+      <FormField
+        control={form.control}
+        name="websiteUrl"
+        render={({ field }) => (
+          <FormItem>
+            <FormLabel>
+              {t("provider.websiteUrl", { defaultValue: "官网链接" })}
+            </FormLabel>
+            <FormControl>
+              <Input {...field} placeholder="https://" />
+            </FormControl>
+            <FormMessage />
+          </FormItem>
+        )}
+      />
+    </>
+  );
+}