feat: 新增应用全局常量 APP，消除硬编码散落

- 新增 src/shared/app.ts，定义应用元信息（name、title、subtitle、description、version）
- 后端 3 处硬编码改为引用 APP.name
- 前端 3 处硬编码改为引用 APP.title/APP.description
- localStorage key 从 my-app.theme.preference 改为 theme.preference
- 构建脚本可执行文件名改为引用 APP.name
- 更新 README.md 和 DEVELOPMENT.md 文档
- 新增 openspec/specs/app-constants/spec.md 规范文档

src/server/config.ts

@@ -1,3 +1,5 @@
+import { APP } from "../shared/app";
+
 export interface ServerConfig {
   host: string;
   port: number;
@@ -43,7 +45,7 @@ export function parseRuntimeArgs(argv: string[] = Bun.argv.slice(2)): { configPa
   if (argv.length === 0) return {};
   const firstArg = argv[0];
   if (firstArg === "--help" || firstArg === "-h") {
-    console.log("用法: my-app [config.yaml]");
+    console.log(`用法: ${APP.name} [config.yaml]`);
     console.log("  config.yaml  可选 YAML 配置文件路径（不存在时使用默认配置）");
     process.exit(0);
   }

src/server/helpers.ts

@@ -1,5 +1,7 @@
 import type { ApiErrorResponse, HealthResponse, RuntimeMode } from "../shared/api";
 
+import { APP } from "../shared/app";
+
 export function createApiError(error: string, status: number): ApiErrorResponse {
   return { error, status };
 }
@@ -18,7 +20,7 @@ export function createHeaders(mode: RuntimeMode, init: HeadersInit): Headers {
 export function createHealthResponse(): HealthResponse {
   return {
     ok: true,
-    service: "my-app",
+    service: APP.name,
     timestamp: new Date().toISOString(),
   };
 }

src/server/server.ts

@@ -2,6 +2,7 @@ import type { RuntimeMode } from "../shared/api";
 import type { ServerConfig } from "./config";
 import type { StaticAssets } from "./static";
 
+import { APP } from "../shared/app";
 import { createApiError, jsonResponse } from "./helpers";
 import { handleHealth } from "./routes/health";
 import { serveStaticAsset } from "./static";
@@ -32,7 +33,7 @@ export function startServer(options: StartServerOptions) {
     },
   });
 
-  console.log(`my-app listening on ${server.url}`);
+  console.log(`${APP.name} listening on ${server.url}`);
 
   return server;
 }