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/EndpointField.tsx

@@ -0,0 +1,63 @@
+import { useTranslation } from "react-i18next";
+import { FormLabel } from "@/components/ui/form";
+import { Input } from "@/components/ui/input";
+import { Zap } from "lucide-react";
+
+interface EndpointFieldProps {
+  id: string;
+  label: string;
+  value: string;
+  onChange: (value: string) => void;
+  placeholder: string;
+  hint: string;
+  showManageButton?: boolean;
+  onManageClick?: () => void;
+  manageButtonLabel?: string;
+}
+
+export function EndpointField({
+  id,
+  label,
+  value,
+  onChange,
+  placeholder,
+  hint,
+  showManageButton = true,
+  onManageClick,
+  manageButtonLabel,
+}: EndpointFieldProps) {
+  const { t } = useTranslation();
+
+  const defaultManageLabel = t("providerForm.manageAndTest", {
+    defaultValue: "管理和测速",
+  });
+
+  return (
+    <div className="space-y-2">
+      <div className="flex items-center justify-between">
+        <FormLabel htmlFor={id}>{label}</FormLabel>
+        {showManageButton && onManageClick && (
+          <button
+            type="button"
+            onClick={onManageClick}
+            className="flex items-center gap-1 text-xs text-gray-600 dark:text-gray-400 hover:text-gray-900 dark:hover:text-gray-100 transition-colors"
+          >
+            <Zap className="h-3.5 w-3.5" />
+            {manageButtonLabel || defaultManageLabel}
+          </button>
+        )}
+      </div>
+      <Input
+        id={id}
+        type="url"
+        value={value}
+        onChange={(e) => onChange(e.target.value)}
+        placeholder={placeholder}
+        autoComplete="off"
+      />
+      <div className="p-3 bg-amber-50 dark:bg-amber-900/20 border border-amber-200 dark:border-amber-700 rounded-lg">
+        <p className="text-xs text-amber-600 dark:text-amber-400">{hint}</p>
+      </div>
+    </div>
+  );
+}