fix(desktop): 补齐 mermaid 放大视图的拖拽区退出 + 把 #772 的守卫换成真实级联探针

#773 修掉了面板宿主层继承宿主 `body > :not(#root) { no-drag }` 导致的 macOS 拖拽
失效，但只覆盖了 `.mermaidModal` 之外的层——而放大视图是插件**第二个铺满视口的
body 直挂层**（`position:fixed; inset:0`，portal 到 body），打开期间会同样清空整窗
拖拽与 macOS 双击标题栏缩放。

- src/client/sidebar.module.css：`.mermaidModal` 加 `-webkit-app-region: initial
  !important`（与面板宿主同一条契约），并在面板宿主那段注释里写明「哪些交互层
  绝对不能加 initial」（`.selectionPopup` 是 `<button>`、FloatingWindow /
  AnchoredPopover 自带指针拖拽，宿主 `:is(button, …)` 已给它们 no-drag；反过来
  声明 `initial !important` 会让按下变成拖窗，重演 #103/#111）。
- tests/panel-host-css.spec.ts：新用例把形状钉在 Linux 的 `pnpm test` 里（宿主层
  与面板层必须是 `initial !important`、`> *` 必须是 `no-drag`、`.mermaidModal`
  同样退出、`.selectionPopup` 不得出现 `initial`）——没有 macOS runner，丢掉
  `!important` 只能靠这里才不会静默复发。
- tests/e2e/mount.e2e.ts：把 #773 的**文本正则**断言换成**真实级联探针**——逐字
  注入宿主两条 darwin 规则、翻转 `data-platform=darwin`，读取计算值并断言
  `[data-window-drag]` 仍 `drag`、宿主层与面板层为 `none`、`[data-dsh-panel-host]
  > *` 为 `no-drag`，随后还原。探针把 `[data-window-drag]` 放在 `#root` 内：直挂
  body 的元素会被宿主那条 id 级规则本身命中（这正是问题机制）。
- docs/external-plugin-guide.md §12.1：按仓库规则同步这条皮肤契约（含交互弹层
  的反例判据与守卫位置）。

实测（scratch profile + 真实 dsh web + Chromium）：
- npm dsh-better-sidebar@0.22.0：探针红，`host` 计算值 `no-drag`（正是线上症状）
- 本分支打包版：探针绿（`strip=drag` / `host=none` / `layer=none` / `child=no-drag`）
- 本地挂载 lane：7 passed（mount + perf，含新探针）

验证：typecheck / lint 通过；panel-host-css + theme 用例通过；新产物
`app-region:initial!important` 2 处（面板宿主 + mermaid 模态）、`app-region:no-drag` 2 处。

src/client/sidebar.module.css

@@ -69,7 +69,15 @@
    shell recomputed regions (window drags worked once, then stopped).
    Take the host and layer out of the app-region computation; `!important`
    because the shell's `body > :not(#root)` selector outranks ours. The
-   panels themselves stay no-drag so their controls keep receiving clicks. */
+   panels themselves stay no-drag so their controls keep receiving clicks.
+   The plugin's OTHER viewport-sized body child — the mermaid zoom modal —
+   carries the same reset on its own rule further down, or opening a diagram
+   would blank window drag (and the macOS double-click-title zoom) while it
+   is up. Deliberately NOT applied to the interactive popups (`.selectionPopup`
+   is a `<button>`; FloatingWindow / AnchoredPopover own pointer drags): the
+   shell's `:is(button, a, input, …)` rule already keeps those no-drag, and
+   `initial !important` would outrank it and start a window drag on press
+   instead — the click-swallow regression of issues #103/#111. */
 :global(html[data-platform] body > [data-dsh-better-sidebar]),
 :global([data-dsh-panel-host]) {
   -webkit-app-region: initial !important;
@@ -1714,6 +1722,15 @@ body[data-dsh-sidebar-dragging] .bottomPanel {
 .mermaidModal {
   position: fixed;
   inset: 0;
+  /* Viewport-sized body child: the shell makes every direct body child
+     `no-drag` (`html[data-platform=darwin] body > :not(#root)`, an
+     id-bearing selector, hence `!important`), and app-region ignores
+     pointer-events — without this reset the open modal cancels every
+     window-drag strip under it plus the macOS double-click-title zoom.
+     Same contract as the panel host rule above; `initial` computes to the
+     neutral `none`, and the toolbar's buttons keep their own no-drag from
+     the shell's `:is(button, …)` rule. */
+  -webkit-app-region: initial !important;
   z-index: 1000;
   display: flex;
   flex-direction: column;