feat: 全局设置系统 — settings 表、CRUD 路由、主题偏好持久化

src/shared/api.ts

@@ -237,11 +237,17 @@ export type ProviderType = "anthropic" | "openai" | "openai-compatible";
 
 export type RuntimeMode = "development" | "production" | "test";
 
+export interface SettingsData {
+  theme: ThemePreference;
+}
+
 export interface TestModelRequest {
   externalId: string;
   providerId: string;
 }
 
+export type ThemePreference = "dark" | "light" | "system";
+
 export interface UpdateModelRequest {
   capabilities?: ModelCapability[];
   contextLength?: null | number;