feat(static-pages): rich text extensions + HTML-mode validation

Milestone 3 of the Static Pages Module sprint.

- MarketplaceHtmlEditorComponent toolbar: horizontal rule
  (insertHorizontalRule), code block (formatBlock -> PRE), embed (prompt for
  a URL, insert a sandboxed <iframe sandbox="allow-scripts allow-same-origin"
  loading="lazy">, same prompt-based UX as the existing link/image commands -
  no new dependency, consistent with the documented no-external-rich-text-
  library decision).
- toggleCode() now validates raw HTML via schema/validators/primitives'
  validateHtml (added in M1) before committing it back to the visual surface;
  on failure it stays in code mode with an inline error instead of silently
  writing malformed markup into the contenteditable surface. Error clears on
  the next edit.
- i18n: builder.promptEmbedUrl, builder.htmlEditorInvalidHtml in interface +
  en/ru/hy.

Gate: tsc --noEmit, npm test (57/57), arch:check, build all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/i18n/en.ts

@@ -551,6 +551,8 @@ export const en: Translations = {
     confirmLeaveUnsaved: 'You have unsaved changes. Leave anyway?',
     promptLinkUrl: 'URL',
     promptImageUrl: 'Image URL',
+    promptEmbedUrl: 'Embed URL',
+    htmlEditorInvalidHtml: 'This HTML has an unclosed or mismatched tag. Fix it before switching back to the visual editor.',
     htmlEditorCode: 'Code',
     htmlEditorPreview: 'Preview',
     lastSaved: 'Last saved',