feat: 服务化 API 新增 urlTarget 外链认领 + 浏览器 HTTP/HTTPS 二级设置开关

基于 PR #74 的颗粒度讨论向前兼容拓展服务化 API：
- TabDescriptor 新增可选 urlTarget(url) 谓词：聊天/界面外链被拦截时按注册
  顺序路由到第一个命中的插件 tab 类型（openTab({ type, url, title })），
  内置 browser 退居隐式兜底；SIDEBAR_FEATURES 追加 'urlTarget'
- 浏览器链接设置改为二级菜单（齿轮弹窗）：新增 browserInterceptHttp
  （默认开）+ browserInterceptHttps（默认关）；browserInterceptLinks
  保留为总闸（默认开，旧文档零迁移）
- 拦截管线：registerLinkInterception 的 takeoverEnabled 改收解析后的
  URL，协议/同源纯判定（shouldInterceptLink）签名不变
- 测试：matchUrlTarget（注册顺序/抛错吞掉/browser 永不命中）、jsdom
  点击接管门控、parsePrefs 新字段默认值；全默认字面量同步
- 文档：设计文档 + AGENTS.md（字段/示例/设置键）+ README（外链分流）

src/client/locales.ts

@@ -166,7 +166,11 @@ export const zh = {
   settingsBrowserSandboxTitle: '关闭浏览器沙箱（不安全）',
   settingsBrowserSandboxDesc: '关闭后，访问的任何网站都将与界面同源运行，可读取会话数据并冒充你的登录状态。仅对完全可信的站点开启',
   settingsBrowserLinksTitle: '聊天区外链在侧边栏打开',
-  settingsBrowserLinksDesc: '开启后，点击聊天或界面中的 http/https 外链时在侧边栏浏览器中打开，不再弹出新窗口；Ctrl/Cmd 点击可临时放行',
+  settingsBrowserLinksDesc: '开启后，点击聊天或界面中的外链时在侧边栏打开，不再弹出新窗口；HTTP 与 HTTPS 可分别通过下方开关控制；Ctrl/Cmd 点击可临时放行',
+  settingsBrowserHttpTitle: '侧边打开HTTP网页',
+  settingsBrowserHttpDesc: '开启后，点击聊天或界面中的 HTTP 外链时在侧边栏打开（声明了 urlTarget 的插件页面优先）；Ctrl/Cmd 点击可临时放行',
+  settingsBrowserHttpsTitle: '侧边打开HTTPS网页',
+  settingsBrowserHttpsDesc: '开启后，点击聊天或界面中的 HTTPS 外链时在侧边栏打开。默认关闭：多数 HTTPS 站点拒绝被嵌入，走系统浏览器更顺畅',
   browserOpenExternal: '在浏览器中打开',
   browserEmbedBlocked: '{host} 拒绝了嵌入请求',
   browserEmbedBlockedDesc: '该站点通过 X-Frame-Options / frame-ancestors 禁止在其它页面中显示，无法在侧边栏内加载。可在浏览器中直接打开',
@@ -378,7 +382,11 @@ export const en: Record<keyof typeof zh, string> = {
   settingsBrowserSandboxTitle: 'Disable browser sandbox (unsafe)',
   settingsBrowserSandboxDesc: 'With the sandbox off, any visited site runs with the same origin as the GUI: it can read session data and act as your logged-in session. Only enable for fully trusted sites',
   settingsBrowserLinksTitle: 'Open chat external links in the sidebar',
-  settingsBrowserLinksDesc: 'When on, clicking an http/https external link in the chat or GUI opens the sidebar browser instead of a new window; Ctrl/Cmd+click always bypasses',
+  settingsBrowserLinksDesc: 'When on, clicking an external link in the chat or GUI opens the sidebar instead of a new window; HTTP and HTTPS are controlled separately by the switches below; Ctrl/Cmd+click always bypasses',
+  settingsBrowserHttpTitle: 'Open HTTP pages in the sidebar',
+  settingsBrowserHttpDesc: 'When on, clicking an HTTP external link in the chat or GUI opens the sidebar (plugin pages declaring urlTarget win); Ctrl/Cmd+click always bypasses',
+  settingsBrowserHttpsTitle: 'Open HTTPS pages in the sidebar',
+  settingsBrowserHttpsDesc: 'When on, clicking an HTTPS external link in the chat or GUI opens the sidebar. Off by default: most HTTPS sites refuse to be embedded, so the system browser is the smoother default',
   browserOpenExternal: 'Open in browser',
   browserEmbedBlocked: '{host} refused to be embedded',
   browserEmbedBlockedDesc: 'The site forbids being displayed inside other pages (X-Frame-Options / frame-ancestors), so it cannot load in the sidebar. Open it directly in your browser instead.',