feat(17-02): richeditor field on TipTap with the untouched rule

- RichEditorField: Reka toolbar, link popover with the http/https/mailto/tel
  and relative URL policy, Edit HTML view, lossy note, skeleton while the
  lazily imported TipTap chunk loads, HTML view fallback when it fails
- untouched values are submitted byte-identical; after an edit the value is
  the editor HTML (empty paragraph -> ""); a value from outside (the save's
  sanitized answer) reloads the editor
- read-only renders through a client display allowlist, never as stored
- TipTap 3.31.4 (core, pm, starter-kit) pinned exact after the package check
- richeditor.* phrase keys (en, pl), smoke specs, forms docs

admin/src/components/form/richeditor/lossy.ts

@@ -0,0 +1,52 @@
+// What the richeditor's TipTap schema can show (UI-SPEC RichEditorField,
+// "Lossy note"). Stored HTML with any other element still opens: the field
+// keeps the stored string until the administrator edits (the untouched
+// rule), and the HTML view edits it without loss. This module has no TipTap
+// import, so the field can decide before the editor chunk loads.
+
+/** Elements the editor shows; compared by lower-cased tag name. */
+export const EDITOR_TAGS: ReadonlySet<string> = new Set([
+  'p',
+  'br',
+  'strong',
+  'b',
+  'em',
+  'i',
+  'u',
+  's',
+  'h3',
+  'ul',
+  'ol',
+  'li',
+  'blockquote',
+  'a',
+  'div',
+  'span',
+])
+
+/**
+ * Parses html into an inert template: nothing in it runs or loads (no
+ * scripts, no image requests), so stored markup can be inspected safely.
+ */
+export function inertFragment(html: string): DocumentFragment {
+  const template = document.createElement('template')
+  template.innerHTML = html
+  return template.content
+}
+
+/** The distinct lower-cased tag names of html's elements, in document order. */
+export function tagNames(html: string): string[] {
+  if (html === '') {
+    return []
+  }
+  const out = new Set<string>()
+  for (const element of Array.from(inertFragment(html).querySelectorAll('*'))) {
+    out.add(element.localName.toLowerCase())
+  }
+  return [...out]
+}
+
+/** Whether html holds an element the editor cannot show (the lossy note). */
+export function isLossy(html: string): boolean {
+  return tagNames(html).some((name) => !EDITOR_TAGS.has(name))
+}