chore(release): 0.15.0 (#309)

- bump version: package.json / dsh.plugin.json / SIDEBAR_SERVICE_VERSION
- README / README_EN: consolidate the two unreleased v0.14.1 sections into
  v0.15.0 covering everything since v0.14.0 (side chat beta, file-window
  upload, desktop-compat four options, settings-page modernization,
  recommended-plugin catalog additions, subagent live batch route, drag and
  push fixes, DSH 0.1.1-rc adaptation, upload hardening); restore the
  historical v0.14.0 section; fix the glued '### v0.13.1### v0.13.1'
  heading in README_EN

dsh.plugin.json

@@ -1,6 +1,6 @@
 {
   "id": "dsh-external/dsh-better-sidebar",
-  "version": "0.14.2",
+  "version": "0.15.0",
   "main": "./lib/index.js",
   "description": "VSCode 风格右侧侧边栏：文件资源管理器 / 编辑器 / 终端 / Git / 浏览器，按会话隔离。暴露服务供其他插件注册侧边栏页面和文件预览器",
   "engines": {