feat: adapt to DSH 0.1.7-alpha.1 and yield the read-only previews

DSH 0.1.7-alpha.1 is 1299 commits past 0.1.6-alpha.2 and rewrites four
contracts this plugin was built on. The peer floor moves to
^0.1.7-alpha.1 (semver's prerelease rule means ^0.1.6-alpha.2 can never
match a 0.1.7 prerelease); 0.1.6-alpha.2 and earlier stay on v0.19.1.
Released as 0.21.0-alpha.1 (dist-tag alpha).

Hard breaks:
- dsh-settings was rewritten: register(ns, schema) is gone, a form is
  addressed by the plugin row's Loader entry id and read from the
  module's exported Config. The preferences are merged into Config and
  marked volatile — which requires DSH's own schemastery, because only
  that build wraps a volatile field in the cosmokit reference the
  loader's volatile commit walks. With the public package a settings
  write reports success and revision 1 while the effective value never
  changes.
- ui-primitives renamed its whole icon family (IconXxx14/16 ->
  IconXxxRegular/Medium): 26 imported symbols, 17 files. The failing
  mode is render-time, not load-time. Five call sites that relied on the
  old 14px default now pass size={14} explicitly.
- Session format v4 rejects source.kind === 'plugin'; the side-chat
  boundary injection now writes plugin:dsh-better-sidebar, the same kind
  DSH's own v3->v4 migration produces, so historical threads survive.
  Tool results moved to role 'tool' with top-level content/isError; all
  four parsers accept both shapes.
- The client session snapshot dropped subagentsByParent/jobsBySession
  and the on-demand subagent-catalog observation API. The Tasks page
  reads projectionsBySession and a new jobs.list route.

Yields and convergence:
- The read-only previews go to DSH's documentpreview (Office->PDF, a
  spreadsheet worker, zoom, per-directory refresh); editor.canOpen
  refuses those 26 extensions. Markdown, HTML (with its sandbox
  switches) and the editable code buffer stay.
- The host browser kind is desktop-profile-only now, so the web e2e
  lanes expect no browser entry and assert its absence.
- Link takeover claims only urlTarget links and passes the rest to the
  host, fixing a regression from the 0.1.6 round: the fallback pointed
  at a 'browser' type the plugin no longer registers, so http links in
  its own markdown did nothing.

Also fixed, found along the way:
- The file tree refreshes live (new directory watch over the plugin's
  own socket), since the plugin owns the `files` kind.
- ctx.sessions.list has no `current` field at either tag; the three
  readers were dead. They now use ctx.sidebarRight.mounted.
- package.json.files was missing lib/client-locale.js.
- Two genuinely dead call sites: sessions.openSubagent/open/
  refreshSubagents do not exist on the current baseline, so the
  topology click, the jump-back and the retry were all no-ops.
- The test harness window shim lacked addEventListener, which the 0.1.7
  primitives call at module-evaluation time; and the e2e token exchange
  needs Playwright's request stack, not Node's fetch.

src/client/locales.ts

@@ -292,11 +292,8 @@ export const zh = {
   settingsGeneralTitle: '常规',
   settingsPopup: '功能设置',
   settingsViewerCatchAll: '兜底：任意文件',
-  viewerImage: '图片',
-  viewerPdf: 'PDF',
   viewerMarkdown: 'Markdown',
   viewerCode: '代码',
-  viewerBinary: '二进制下载',
   viewerHtml: 'HTML',
   browser: '浏览器',
   browserPlaceholder: '输入网址，例如 example.com',
@@ -317,12 +314,6 @@ export const zh = {
   settingsHtmlSandboxDesc: '关闭后，预览的 HTML 将与界面同源运行，可读取会话文件、本地存储并调用内部接口。仅对完全可信的文件开启',
   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',
   settingsBrowserLoopbackTitle: '允许访问的本机地址',
   settingsBrowserLoopbackDesc: '逗号分隔的本地回环地址白名单（如 localhost:5174 或 127.0.0.1:8080），侧边栏浏览器可访问这些本地服务；默认留空则本机地址全部拦截。沙箱隔离仍然生效，页面无法读取界面数据',
   settingsBrowserLoopbackPlaceholder: '例如 localhost:5174, 127.0.0.1:8080',
@@ -734,11 +725,8 @@ export const en: Record<keyof typeof zh, string> = {
   settingsGeneralTitle: 'General',
   settingsPopup: 'Feature settings',
   settingsViewerCatchAll: 'Catch-all: any file',
-  viewerImage: 'Image',
-  viewerPdf: 'PDF',
   viewerMarkdown: 'Markdown',
   viewerCode: 'Code',
-  viewerBinary: 'Binary download',
   viewerHtml: 'HTML',
   browser: 'Browser',
   browserPlaceholder: 'Enter a URL, e.g. example.com',
@@ -759,12 +747,6 @@ export const en: Record<keyof typeof zh, string> = {
   settingsHtmlSandboxDesc: 'With the sandbox off, previewed HTML runs with the same origin as the GUI: it can read session files, local storage and call internal APIs. Only enable for fully trusted files',
   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 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',
   settingsBrowserLoopbackTitle: 'Allowed local addresses',
   settingsBrowserLoopbackDesc: 'Comma-separated allowlist of loopback addresses (e.g. localhost:5174 or 127.0.0.1:8080) the sidebar browser may visit; empty blocks all local addresses by default. The sandbox still applies — pages cannot read GUI data',
   settingsBrowserLoopbackPlaceholder: 'e.g. localhost:5174, 127.0.0.1:8080',