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/ApiKeyInput.tsx

@@ -0,0 +1,72 @@
+import React, { useState } from "react";
+import { Eye, EyeOff } from "lucide-react";
+import { useTranslation } from "react-i18next";
+
+interface ApiKeyInputProps {
+  value: string;
+  onChange: (value: string) => void;
+  placeholder?: string;
+  disabled?: boolean;
+  required?: boolean;
+  label?: string;
+  id?: string;
+}
+
+const ApiKeyInput: React.FC<ApiKeyInputProps> = ({
+  value,
+  onChange,
+  placeholder,
+  disabled = false,
+  required = false,
+  label = "API Key",
+  id = "apiKey",
+}) => {
+  const { t } = useTranslation();
+  const [showKey, setShowKey] = useState(false);
+
+  const toggleShowKey = () => {
+    setShowKey(!showKey);
+  };
+
+  const inputClass = `w-full px-3 py-2 pr-10 border rounded-lg text-sm transition-colors ${
+    disabled
+      ? "bg-gray-100 dark:bg-gray-800 border-gray-200 dark:border-gray-700 text-gray-400 dark:text-gray-500 cursor-not-allowed"
+      : "border-gray-200 dark:border-gray-700 dark:bg-gray-800 dark:text-gray-100 focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 focus:border-blue-500 dark:focus:border-blue-400"
+  }`;
+
+  return (
+    <div className="space-y-2">
+      <label
+        htmlFor={id}
+        className="block text-sm font-medium text-gray-900 dark:text-gray-100"
+      >
+        {label} {required && "*"}
+      </label>
+      <div className="relative">
+        <input
+          type={showKey ? "text" : "password"}
+          id={id}
+          value={value}
+          onChange={(e) => onChange(e.target.value)}
+          placeholder={placeholder ?? t("apiKeyInput.placeholder")}
+          disabled={disabled}
+          required={required}
+          autoComplete="off"
+          className={inputClass}
+        />
+        {!disabled && value && (
+          <button
+            type="button"
+            onClick={toggleShowKey}
+            className="absolute inset-y-0 right-0 flex items-center pr-3 text-gray-500 dark:text-gray-400 hover:text-gray-900 dark:hover:text-gray-100 transition-colors"
+            aria-label={showKey ? t("apiKeyInput.hide") : t("apiKeyInput.show")}
+          >
+            {showKey ? <EyeOff size={16} /> : <Eye size={16} />}
+          </button>
+        )}
+      </div>
+    </div>
+  );
+};
+
+export default ApiKeyInput;