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/useSettingsForm.ts

@@ -0,0 +1,158 @@
+import { useCallback, useEffect, useRef, useState } from "react";
+import { useTranslation } from "react-i18next";
+import { useSettingsQuery } from "@/lib/query";
+import type { Settings } from "@/types";
+
+type Language = "zh" | "en";
+
+export type SettingsFormState = Omit<Settings, "language"> & {
+  language: Language;
+};
+
+const normalizeLanguage = (lang?: string | null): Language => {
+  if (!lang) return "zh";
+  return lang === "en" ? "en" : "zh";
+};
+
+const sanitizeDir = (value?: string | null): string | undefined => {
+  if (!value) return undefined;
+  const trimmed = value.trim();
+  return trimmed.length > 0 ? trimmed : undefined;
+};
+
+export interface UseSettingsFormResult {
+  settings: SettingsFormState | null;
+  isLoading: boolean;
+  initialLanguage: Language;
+  updateSettings: (updates: Partial<SettingsFormState>) => void;
+  resetSettings: (serverData: Settings | null) => void;
+  readPersistedLanguage: () => Language;
+  syncLanguage: (lang: Language) => void;
+}
+
+/**
+ * useSettingsForm - 表单状态管理
+ * 负责：
+ * - 表单数据状态
+ * - 表单字段更新
+ * - 语言同步
+ * - 表单重置
+ */
+export function useSettingsForm(): UseSettingsFormResult {
+  const { i18n } = useTranslation();
+  const { data, isLoading } = useSettingsQuery();
+
+  const [settingsState, setSettingsState] = useState<SettingsFormState | null>(
+    null,
+  );
+
+  const initialLanguageRef = useRef<Language>("zh");
+
+  const readPersistedLanguage = useCallback((): Language => {
+    if (typeof window !== "undefined") {
+      const stored = window.localStorage.getItem("language");
+      if (stored === "en" || stored === "zh") {
+        return stored;
+      }
+    }
+    return normalizeLanguage(i18n.language);
+  }, [i18n.language]);
+
+  const syncLanguage = useCallback(
+    (lang: Language) => {
+      const current = normalizeLanguage(i18n.language);
+      if (current !== lang) {
+        void i18n.changeLanguage(lang);
+      }
+    },
+    [i18n],
+  );
+
+  // 初始化设置数据
+  useEffect(() => {
+    if (!data) return;
+
+    const normalizedLanguage = normalizeLanguage(
+      data.language ?? readPersistedLanguage(),
+    );
+
+    const normalized: SettingsFormState = {
+      ...data,
+      showInTray: data.showInTray ?? true,
+      minimizeToTrayOnClose: data.minimizeToTrayOnClose ?? true,
+      enableClaudePluginIntegration:
+        data.enableClaudePluginIntegration ?? false,
+      claudeConfigDir: sanitizeDir(data.claudeConfigDir),
+      codexConfigDir: sanitizeDir(data.codexConfigDir),
+      language: normalizedLanguage,
+    };
+
+    setSettingsState(normalized);
+    initialLanguageRef.current = normalizedLanguage;
+    syncLanguage(normalizedLanguage);
+  }, [data, readPersistedLanguage, syncLanguage]);
+
+  const updateSettings = useCallback(
+    (updates: Partial<SettingsFormState>) => {
+      setSettingsState((prev) => {
+        const base =
+          prev ??
+          ({
+            showInTray: true,
+            minimizeToTrayOnClose: true,
+            enableClaudePluginIntegration: false,
+            language: readPersistedLanguage(),
+          } as SettingsFormState);
+
+        const next: SettingsFormState = {
+          ...base,
+          ...updates,
+        };
+
+        if (updates.language) {
+          const normalized = normalizeLanguage(updates.language);
+          next.language = normalized;
+          syncLanguage(normalized);
+        }
+
+        return next;
+      });
+    },
+    [readPersistedLanguage, syncLanguage],
+  );
+
+  const resetSettings = useCallback(
+    (serverData: Settings | null) => {
+      if (!serverData) return;
+
+      const normalizedLanguage = normalizeLanguage(
+        serverData.language ?? readPersistedLanguage(),
+      );
+
+      const normalized: SettingsFormState = {
+        ...serverData,
+        showInTray: serverData.showInTray ?? true,
+        minimizeToTrayOnClose: serverData.minimizeToTrayOnClose ?? true,
+        enableClaudePluginIntegration:
+          serverData.enableClaudePluginIntegration ?? false,
+        claudeConfigDir: sanitizeDir(serverData.claudeConfigDir),
+        codexConfigDir: sanitizeDir(serverData.codexConfigDir),
+        language: normalizedLanguage,
+      };
+
+      setSettingsState(normalized);
+      syncLanguage(initialLanguageRef.current);
+    },
+    [readPersistedLanguage, syncLanguage],
+  );
+
+  return {
+    settings: settingsState,
+    isLoading,
+    initialLanguage: initialLanguageRef.current,
+    updateSettings,
+    resetSettings,
+    readPersistedLanguage,
+    syncLanguage,
+  };
+}