新建命令时按执行方式填上默认图标

MCP 建出来的命令 features.icon 是空的，命令卡片上就是个空位。现在 icon 留空时
按 program 填对应语言的默认图标（shell -> logo/shell.png、python ->
logo/python.png，编排用内置图标），显式传了 icon 就用传入的。

test/commandStore.test.js

@@ -45,6 +45,22 @@ const code = command.features.code;
 assert.match(code, /^key_[0-9a-f]{8}$/);
 assert.strictEqual(command.features.cmds[0], "测试命令");
 assert.strictEqual(command.features.mainHide, false);
+// 不带图标时用该执行方式的默认图标
+assert.strictEqual(command.features.icon, "logo/quickcommand.png");
+const pyCommand = store.create({
+  program: "python",
+  cmd: "print(1)",
+  features: { explain: "py", cmds: ["py"] },
+}).command;
+assert.strictEqual(pyCommand.features.icon, "logo/python.png");
+const customCommand = store.create({
+  program: "custom",
+  cmd: "x",
+  features: { explain: "自定义", cmds: ["自定义"], icon: "data:image/png;base64,xxx" },
+}).command;
+assert.strictEqual(customCommand.features.icon, "data:image/png;base64,xxx");
+store.remove(pyCommand.features.code);
+store.remove(customCommand.features.code);
 assert.ok(db.has("qc_" + code));
 assert.ok(features.has(code));