refactor: split useSettings hook into specialized hooks

Before optimization:
- useSettings.ts: 516 lines (single monolithic hook)

After optimization:
- useSettingsForm.ts: 158 lines (form state management)
- useDirectorySettings.ts: 297 lines (directory management)
- useSettingsMetadata.ts: 95 lines (metadata management)
- useSettings.ts: 215 lines (composition layer)
- Total: 765 lines (+249 lines, but with clear separation of concerns)

Benefits:
✅ Single Responsibility Principle: each hook focuses on one domain
✅ Testability: independent hooks are easier to unit test
✅ Reusability: specialized hooks can be reused in other components
✅ Maintainability: reduced cognitive load per file
✅ Zero breaking changes: SettingsDialog auto-adapted to new interface

Technical details:
- useSettingsForm: pure form state + language sync
- useDirectorySettings: directory selection/reset + default value computation
- useSettingsMetadata: config path + portable mode + restart flag
- useSettings: composition layer + save logic + reset logic

src/hooks/useSettingsMetadata.ts

@@ -0,0 +1,95 @@
+import { useCallback, useEffect, useState } from "react";
+import { useTranslation } from "react-i18next";
+import { toast } from "sonner";
+import { settingsApi } from "@/lib/api";
+
+export interface UseSettingsMetadataResult {
+  configPath: string;
+  isPortable: boolean;
+  requiresRestart: boolean;
+  isLoading: boolean;
+  openConfigFolder: () => Promise<void>;
+  acknowledgeRestart: () => void;
+  setRequiresRestart: (value: boolean) => void;
+}
+
+/**
+ * useSettingsMetadata - 元数据管理
+ * 负责：
+ * - configPath（配置文件路径）
+ * - isPortable（便携模式）
+ * - requiresRestart（需要重启标志）
+ * - 打开配置文件夹
+ */
+export function useSettingsMetadata(): UseSettingsMetadataResult {
+  const { t } = useTranslation();
+
+  const [configPath, setConfigPath] = useState("");
+  const [isPortable, setIsPortable] = useState(false);
+  const [requiresRestart, setRequiresRestart] = useState(false);
+  const [isLoading, setIsLoading] = useState(true);
+
+  // 加载元数据
+  useEffect(() => {
+    let active = true;
+    setIsLoading(true);
+
+    const load = async () => {
+      try {
+        const [appConfigPath, portable] = await Promise.all([
+          settingsApi.getAppConfigPath(),
+          settingsApi.isPortable(),
+        ]);
+
+        if (!active) return;
+
+        setConfigPath(appConfigPath || "");
+        setIsPortable(portable);
+      } catch (error) {
+        console.error(
+          "[useSettingsMetadata] Failed to load metadata",
+          error,
+        );
+      } finally {
+        if (active) {
+          setIsLoading(false);
+        }
+      }
+    };
+
+    void load();
+    return () => {
+      active = false;
+    };
+  }, []);
+
+  const openConfigFolder = useCallback(async () => {
+    try {
+      await settingsApi.openAppConfigFolder();
+    } catch (error) {
+      console.error(
+        "[useSettingsMetadata] Failed to open config folder",
+        error,
+      );
+      toast.error(
+        t("settings.openFolderFailed", {
+          defaultValue: "打开目录失败",
+        }),
+      );
+    }
+  }, [t]);
+
+  const acknowledgeRestart = useCallback(() => {
+    setRequiresRestart(false);
+  }, []);
+
+  return {
+    configPath,
+    isPortable,
+    requiresRestart,
+    isLoading,
+    openConfigFolder,
+    acknowledgeRestart,
+    setRequiresRestart,
+  };
+}