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

scripts/build.ts

@@ -2,10 +2,12 @@ import { readdir, rm, writeFile } from "node:fs/promises";
 import { join, relative, sep } from "node:path";
 import { fileURLToPath } from "node:url";
 
+import { APP } from "../src/shared/app";
+
 const projectRoot = fileURLToPath(new URL("..", import.meta.url));
 const distWebDir = join(projectRoot, "dist/web");
 const buildDir = join(projectRoot, ".build");
-const executablePath = join(projectRoot, "dist/my-app");
+const executablePath = join(projectRoot, `dist/${APP.name}`);
 
 async function build() {
   try {