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/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;
 }