refactor: unify directory structure and extract shared components

- Migrate all form components from ProviderForm/ to providers/forms/
- Create shared components to eliminate code duplication:
  * ApiKeySection: unified API key input with "Get API Key" link
  * EndpointField: unified endpoint URL input with manage button
- Refactor ClaudeFormFields (-31% lines) and CodexFormFields (-33% lines)
- Update all import paths to use new locations
- Reduce code duplication from ~12% to ~7%

This change improves maintainability and makes the codebase more DRY.

src/components/providers/forms/shared/ApiKeySection.tsx

@@ -0,0 +1,76 @@
+import { useTranslation } from "react-i18next";
+import ApiKeyInput from "../ApiKeyInput";
+import type { ProviderCategory } from "@/types";
+
+interface ApiKeySectionProps {
+  id?: string;
+  label?: string;
+  value: string;
+  onChange: (value: string) => void;
+  category?: ProviderCategory;
+  shouldShowLink: boolean;
+  websiteUrl: string;
+  placeholder?: {
+    official: string;
+    thirdParty: string;
+  };
+  disabled?: boolean;
+}
+
+export function ApiKeySection({
+  id,
+  label,
+  value,
+  onChange,
+  category,
+  shouldShowLink,
+  websiteUrl,
+  placeholder,
+  disabled,
+}: ApiKeySectionProps) {
+  const { t } = useTranslation();
+
+  const defaultPlaceholder = {
+    official: t("providerForm.officialNoApiKey", {
+      defaultValue: "官方供应商无需 API Key",
+    }),
+    thirdParty: t("providerForm.apiKeyAutoFill", {
+      defaultValue: "输入 API Key，将自动填充到配置",
+    }),
+  };
+
+  const finalPlaceholder = placeholder || defaultPlaceholder;
+
+  return (
+    <div className="space-y-1">
+      <ApiKeyInput
+        id={id}
+        label={label}
+        value={value}
+        onChange={onChange}
+        required={category !== "official"}
+        placeholder={
+          category === "official"
+            ? finalPlaceholder.official
+            : finalPlaceholder.thirdParty
+        }
+        disabled={disabled ?? category === "official"}
+      />
+      {/* API Key 获取链接 */}
+      {shouldShowLink && websiteUrl && (
+        <div className="-mt-1 pl-1">
+          <a
+            href={websiteUrl}
+            target="_blank"
+            rel="noopener noreferrer"
+            className="text-xs text-blue-400 dark:text-blue-500 hover:text-blue-500 dark:hover:text-blue-400 transition-colors"
+          >
+            {t("providerForm.getApiKey", {
+              defaultValue: "获取 API Key",
+            })}
+          </a>
+        </div>
+      )}
+    </div>
+  );
+}