feat(changes): HTML render preview for .html op targets

.html/.htm/.xhtml op previews (read / write / edit, non-error) gain a
Render/Raw toggle in the preview pane head: the render view loads the
SAVED file through the same /sidebar/html route the editor's html viewer
uses, so relative assets (./style.css, img/x.png) resolve inside the
route and a segmented read still renders the whole document (the route
serves the file, not the op snapshot — the file-trace-style srcDoc
approach would render a cut-off document whose scripts never run).

The frame is ALWAYS sandboxed (HTML_IFRAME_SANDBOX, moved to the shared
html-preview.ts module — opaque origin, no allow-same-origin, no
top-navigation) plus the route's CSP sandbox header as
defense-in-depth; unlike the editor viewer this surface has no
no-sandbox escape hatch. The contract is pinned by new sandbox-spec
tests on the extracted HtmlRenderPreview component and the pane toggle
(off by default; absent for non-html and error targets).

Locales: zh/en + all 19 shipped third-language overrides (key-set-equal
gate green). Typecheck + eslint clean; sandbox/redact/diff-rows/frontmatter
suites 36/36 green. The full-suite host-side failures (fs-operations /
pty smoke / git / install-powershell) are pre-existing on the base
branch (verified via stash) and unrelated.

src/client/html-preview.ts

@@ -0,0 +1,15 @@
+/**
+ * Shared vocabulary of the sidebar's HTML preview surfaces (the editor's
+ * html viewer and the changes tab's op render preview): the sandbox tokens
+ * both preview iframes load with.
+ */
+
+/**
+ * The sandbox tokens of an HTML preview iframe. NO allow-same-origin (the
+ * preview must stay in an opaque origin — with the route's own origin it
+ * could read session data) and NO allow-top-navigation (a previewed page
+ * must not hijack the GUI). The editor viewer can disable the sandbox
+ * per-feature in the side card settings (warned); the changes tab's op
+ * render preview always stays sandboxed.
+ */
+export const HTML_IFRAME_SANDBOX = 'allow-scripts allow-popups allow-downloads allow-modals'